Btw, this is on a Slim 360 I purchased maybe 6 months ago.
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This happens at random. It isn't usually something you do it just happens, I also noticed you were playing FoNV and that game does freeze more often than others.
Have you tried the basic steps in the "xbox keeps freezing" thread that is in the console section of this forum? Stuff like clearing your cache can help. As inferior said New vegas is iffy at times.