This has been gone over before, and I don't think it should be like the way you suggest. There was a bug before that the AI didn't take into account of taking upon the ongoing hostilities/war(s) of the vassalized nation, I understand this was fixed (in Warlords?) and the AI is now aware that it will take on those wars?
So assuming the above is true, the AI is offering to be vassalized to another for help in their current war. I see this as logical so why need the change? It's like them offering a tech/gold to for war assistance.
A Civ
can't beg to become a Vassal -- that request doesn't even exist in the Diplomacy interface. A stronger Civ can only demand that a weaker Civ Capitulate to them and become their Vassal.
The problem is that often if a Civ is near defeat a stronger Civ requests that they Capitulate and become their Vassal, but instead of imposing peace they're dragged into the war. The only exception is if the Master Civ was also at war with the Vassal State, in which case when the enemy Civ capitulates it ends the war
What should occur when a Civ Capitulates to another is that an immediate Armistice is declared and a pop-up appears explaining that this Civ has become the Vassal of that Civ; then it should give the Player the option to either declare war on the Master Civ or agree to a Cease Fire.
But this is getting off topic, because I was merely explaining WHY I was compelled to use nukes. This thread is about the realistic effects of nukes, aside from creating fallout, which is a no-brainer -- and is cleaned up far too easily IMO.
In regards to Nuclear Winter, I'm not talking about the far-fetched post-apocalyptic Nuclear Winter that looks like an Ice Age. A plausible Nuclear Winter is actually a world shrouded in perpetual, impenetrable cloud cover, which would obscure direct sunlight and hinder agriculture, especially the farm crops that feed the majority of the world's population. Yes, the climate would be colder because of the lack of sunlight, but it wouldn't mean 100-feet of snow and ice covering the world. In the past, massive volcanic eruptions have cast enough ash and soot into the atmosphere to effect the global climate, which have destroyed entire civilizations. A nuclear war would have a similar effect. The severity and duration of the nuclear winter would depend on the scale of the nuclear exchange. In reality, tactical nukes probably wouldn't cause a nuclear winter at all, because by their very design they're intended to be relatively small battlefield weapons (I've posted an idea of limiting the destruction and fallout of Tactical Nukes to a single map tile, but this wasn't implemented in BtS either), whereas ICBMs are strategic weapons whose primary purpose is deterence and the M.A.D. of a nuclear holocaust. Therefore, ICBMs should cause nuclear winter, but Nukes simply don't have the power on par with a super-volcano's explosive eruption.
As for the issue of Global Warming and the insanely ignorant juvenile bullsh!t notion that humans can't positively effect the planet because "we're so small and the Earth is so big" -- this sounds like a kindergardener's excuse! That's like saying that a drop of water is
harmless; but millions of raindrops can cause flooding, erosion, and mudslides, which kill thousands of people every year. The same analogy applies to humans; individually our effect on the Earth is just a drop in the bucket, but 7+ billion raindrops has and will continue to change the face of our planet for the worse.
By simple deduction, if there are ice ages then there must be some system of global warming in place or we'd all be sitting in our woolies right now (if we were lucky enough to have survived).
and/or increase in 
for every city on the globe)
