I will give you an example:
- original VB4 thread:/threads/5755-Cranesong-Analog-Dither
- new Xenforo thread: /threads/cranesong-analog-dither.4940/
The potential problem I see is that both VB4 and Xenforo use "threads" in the url structure.
One of the ideas I had was to change it to "thread" in Xenforo. Then the redirection script for VB3 would work fine:
- original VB4 thread:/threads/5755-Cranesong-Analog-Dither redirects to /showthread.php?t=5755
- /showthread.php?t=5755 redirects to Xenforo location /thread/cranesong-analog-dither.4940/
Another solution would be to install Xenforo in a sub directory. But I think it looks better for google if the Xenforo is in the server's root directory, doesn't it?