[Reeve of shinra]

Common problem... but you hit the nail on the head in its difficulty.

While I haven't had a need to do this myself, the two common suggested solutions are:

a) hack either the forums or your sites log in script to update both sets of tables when changes are made

b) hack your site to use the vbulletin user table instead.

This is a little beyond my skills but there is a basic soap framework floating around here somewhere that you might be of some use to you.