Just tested your context.vim under vim 6.3.x; It's better than tex/latex standard syntax in vim 6.3 IMHO.
I have only a small remark: The following code is valid in ConTeXt:
\useURL [book-ns-openssl][http://www.amazon.com/exec/obidos/ASIN/ 059600270X/qid%3D1024342376/103-8123753-9606263][Network Security with OpenSSL]
but the syntax considers % like a comment mark, so I see another color for the text after %. It's possible to make the comment pattern smarter ? :)
Hm, probably not. The problem with TeX is that it's so goddam flexible when it comes to "syntax". The whole deal with turning $ on and off, % into something else, and so on is really hard to capture in a syntax highlighter. To do so successfully, the highlighter would more or less have to parse the TeX document itself to understand when and where to do what. I guess special cases like the one above (with a very weird URL indeed) so that \useURL would be parsed differently, but it's impossible to always get these things right, nikolai