Things are valuable insofar as someone decides they are valuable. Most animals are not conscious of their physical or physchological state. This means there is no prima facie obligation not to damage their physical or physchological state. Animals don't make decisions about their lives, decisions are what confer value, therefore animals lack value and rights.
People might value them. This is why it is important that animals not unduly suffer pain and hardship where it is easy for them not to. It is not some naturalistic pain/pleasure based rubric that results in rights and duties. If people value cheap hamburgers over happy cows then the moral thing to do is respect those values, insofar as doing so does not limit the ability of other decision-making creatures to make important decisions about their lives. On that last point the environmental vegetarian might have a case.
Rule Pluralism.
Values are not important because they are merely the outcomes of decisions. They are important because we have the ability to deliberate between life-projects and through this deliberation ascribe them. Value ascription requires at least the potential to make an assessment about some circumstance, compare it to another and select the one we deem valuable. That process is why we should care about ethics. It means that some individual is purposive and not just a consequence of molecules smashing together in contingent arrangements.
This purposiveness is why my appeal to introspection is not arbitrary or question begging, as Traitorfish accuses me of above. I can answer the question of why we should care about treating anything with respect at all since we live in an accidental and objectively meaningless world. The utilitarian pain/pleasure/suffering rubric cannot; it is always subject to the question of why I should take pain and pleasure, which are accidental evolutionary constructs, to map perfectly on to the abstract ethical concepts of good and bad. My introspection/autonomy based ethical theory can also account for disagreement in way that utilitarianism cannot. As I've said already, some consider pain to be ethically important and not merely as a means to obtain higher pleasure.
Your thinking seems uncharacteristically confused here. You seem to be (and correct me if I'm wrong) putting forward two accounts of ethics in different fields, and I do not believe that they are compatible. I do not think that your metaethical* views are compatible with your normative* views.
To whit, you put forward the metaethical view that "things are valuable insofar as someone decides they are valuable." This seems like a relativistic account of value; the intrinsic value of things is relative to whether people value them. It is only when people make a positive evaluative decision (a decision that something is valuable) that something acquires value. To put it another way, the truth value of sentences of the form 'X is valuable' (sentences that predicate 'valuable' to an object) is dependent on whether people believe 'X is valuable' or do not believe' X is valuable'.
On this theory, something is only valuable if people value it. 'People' are those being that are introspective and autonomous to a sufficient level. Things like car crashes or good health and are disvaluable or valuable to the extent that people value them. If people stopped valuing good health good health would be disvaluable and if people started valuing car crashes car crashes would be valuable. The value of a thing is relative to whether people value it.
However your second, normative, view seems to pull in a different direction entirely. You start saying that "[Values] are important", that we 'should care about ethics' and that you can answer 'why should we care about treating anything with respect at all?'. You seem to be ascribing universal importance to something (value) and claiming that there are universal ethical dictums. In parts you seem to imply that decisions themselves are of intrinsic value and in parts that introspection and autonomy are of intrinsic value.
The best I can re-construct your theory here is at least as yielding the claims that animals, because they lack introspection, autonomy and appropriate decision making mechanisms, are not valuable. They do not have the ability to do anything valuable and they do not display any valuable properties. Humans, because they can do these things, are valuable. The normative view you put forward is something like the view that the activity of deciding in an autonomous introspective way is the
only thing that is valuable or that confers value.
I think it is clear that the latter view conflicts the former view. On the former view 'autonomous introspection' is valuable if and only if people thing it is valuable. It is the only thing that is valuable if and only if is the
only thing people value. But it is simply not the case that people only value autonomous introspection. People value a whole host of other things; art, family, children, literature, personal pleasure so on and so forth. It is not even obvious that autonomous introspection is the thing people value most. On your metaethical view autonomous introspection cannot possible have the pride of place your normative view gives it. Your two views are incompatible; they contradict each other.
You must drop one of your theories if you want your beliefs to be consistent. There is little credibility in your conclusion (of the moral non-obligatoriness of vegetarianism) if your beliefs are inconsistent. However, I do not think you will be able to do so in a way which will yield the conclusion you have put forward.
You could, for instance, drop your normative theory. This might leave you with the metaethical theory that something is valuable if autonomous introspective beings believe it to be of value. Consequently, humans decide which things are of value and which things of disvalue. Animals do not. When a human has decided that something is of value, it is of value.
But I doubt this will yield the appropriate conclusions. I find it quite obvious that people do value animal life and do find pain simpliciter of disvalue. That they value this would commit you to the conclusion that animal life is of intrinsic value. Quite plausibly, in reflective equilibrium most people would find the pain and suffering caused by the meat industry to be not worth the benefits.
Indeed, I find this a rather implausible metaethical theory regardless. It clashes with too many of our intuitions. It means that vengeance killing is justified if people think it is, that in a society in which slavery is accepted by all (even by the poor slaves) slavery is permitted and that, in general, when peoples opinions change the morality of even the most heinous action changes.
The alternative is to stick be the normative theory and drop the metaethical theory. That is, to say that autonomous introspection is the only thing of value and to have an undefined metaethics. This would probably yield the conclusions you come to; if we accept animals lack the appropriate level of autonomous introspection their lives are of only instrumental value.
But this theory is thoroughly implausible. Although autonomous introspection is almost certainly of value, it is equally certainly not the only thing of value. The most vivid cases probably concern pain; I think it is quite clear that a world in which thousands of dogs are tortured needlessly is a worse world than that in which they are not. The pain, or the torture, has disvalue. In general, our belief that needlessly harming animals is wrong is dependent on the view that suffering, even if undergone by an animal, is of disvalue.
The same point has been raised as regards babies. Babies are not autonomous and there is little reason to believe they are introspective. Yet, it is almost always immoral to hurt or hit a baby, let alone kill a baby. If autonomous introspection were the only thing of value, this would not be the case. A babies life or well-being would have at best instrumental value, rather than the intrinsic value we ascribe it.
If we have to include suffering and pain in our first-order accounts of value I think we will have difficulty proclaiming the morality of much of the modern meat industry.
*That is to say, metaethical when combined with a consequentialist moral theory. Else it is just a 'meta-evaluative' view with no necessary ethical implications.
*And again, normative only when combined with consequentialism. Otherwise it is simply an account of value. 
