Sorry man... VBA Dynamics is a commercial mod I just don't have access to so I don't know how it's designed or what fields may be available.
If there is a php hook available somewhere you could make a plugin with this code, it will print out all available fields for $entry and see if any of them match the userid.
Code:
echo "<pre>";
print_r ($entry);
echo "</pre>";
But don't do it while your forum is on (use a test forum if you have one) otherwise everyone will see the code output.