Ok- here's the final analysis....

The converter from YaBB to phpbb is not working correctly. It is dumping some users from the transition with errors, I can only imagine what it might be doing to posts...

So I'm fairly sure that a stock transition from phpbb to vbulletin would go without a hitch, the problem lies with the yabb conversion to phpbb.

A couple of further notes:

My YaBB sp1 install converted flawlessly into YaBB SE. Although I see no converters around for YaBB SE to anything at this point.

Seriously, I would write my own conversion, but really my programming skils (as modest as they are) are not up to the task.

I would think that a conversion from YaBB SE to vbulletin would be fairly painless since you are working with data coming out of the same database engine. But again it's my horrible programming skills stopping me (and I have to make sure I don't ignore my clients).

Thanks for the help... however, this is a dead end conversion path. Looks like I'm landlocked on my current system since I can lose the content...

Guess I'll have to sit on my vbulletin license and hope a solution comes up.

Sadly, I would get hung by my board of directors (we are a NPC) if I lost the content....

<weep>