A UBB 6.3.1 to vBulletin is possible as I've just recently done this myself.
At the moment considering that there's no import scripts for vB3 what I did was to install vB2 and then import UBB, I then upgraded by vB2 to vB3. This allowed me to have a successful conversion between the two bulletin board systems.
As to the Attachment limits, you can set an overall limit of attachment files within the vB Settings, however an individually/usergroup defined limit will almost certainly require hacking.
As to allowing members to delete older attachments, this is already a feature within vB3.