As far as I can tell, the only thing they are modifying is th field that says what vendor's phone is allowed to install it
Of course a quick deletion of the vendor.xml fixes all problems.
I'm not an expert, but when I've been comparing files here and there in the different vendors releases, it just doesn't seem to me they are really doing anything.
The one code upgrade that really made a difference for me was the 7230 .185 that TMobile deployed, I found it bug ridden with using the Address Book, so I said, screw it, and I put on the .188 code, after doing this, almost every problem disappeared.
I first got into this habit when I first got my 7100t, the .112 code on it was so bad, I'd be getting a java error almost each time I did anything, when I stumbled upon the forums, I read that Vodafone had released the .128 code and TMobile was still "testing". So voila, .128 installed and almost all the problems went away.