With 64-bit vfw codec for xvid (you can download one from xvid.org) you can get partially compatible input in 64-bit Vegas. Generally you'll see picture and in simple cases it will work well, but some files can give you green screen at the beginning, and sometimes picture will be simply crushed by square artifacts.
The cause of issue is in incompatibility of video for windows program interface and avi container with some features of mpeg-4 part 2 codecs. Also there is lack of support of vfw "hack" (packed streams) that can deal with issues in other programs.
Safest way is to reencode file and never push such files into Vegas directly.