First step is to make sure you are using the latest stable version of vitrualdub.
There are a ton of possible problems with AV sync. Off the top of my head, the most probable problems are differing framerates and/or audio sampling frequencies, and erroneous timestamp indexing. Go to doom9.org and search their forums for "avi offset calculator". Assuming that your divx file is using an .avi wrapper, run the calculator to see if the time stamp is synced up properly.
Also, you could try reencoding one or both files to a common framerate and audio sampling rate to see if that resolves the issue. I know the quality will suffer from reencoding twice, but if you find that is the problem, you could at least move on to another program to work with the source files before importing into vdub.