You seem to be under the impression that just because Humberto Leal was walking trash, the government of Texas shouldn't have tried to give him a fully fair trial, and informed him of his legal rights. Justice doesn't work that way. Trespassers and murderers both get a fully fair trial, no exceptions. Also, you're a little confused. The United States, and Texas in general, does fall under the Vienna Convention. We signed and ratified the thing after all. The problem is that the Supreme Court may a weird reading that Congress not only had to ratify a treaty, but also make a separate law announcing that it would be enforced. So the treaty applies to this case, there's just no enforcement mechanism.