This issue comes up now and again with Vegas x64 and legacy 32 bit VFW codecs. I think I posted this before, but if so, I apologize. The same person that developed the AMV codec (amamaman) also developed a utility called ProxyCodec64 (scroll to link at page bottom):

http://amamaman.hp.infoseek.co.jp/english/top_e.html

ProxyCodec64 "fools" 64 bit software into using 32 bit video codecs. It only works with VFW codecs, but it can be a handy tool when you want to use Vegas 64 and there is no 64 bit equivalent for your legacy codec.

I haven't used it in a while (since Cineform provided a 64 bit version of its codec), but it may still be useful.