Being an Oracle Developer myself and having recently learned PHP. I can say that each and every SQL statement in the newest version (1.1.4) will have to be changed to support ORACLE.

Especially getting rid of those JOINs.

The actual structure of the database is ok, except for the auto-increment fields but sequences will handle that.

Given That:
If you have a good developer, he should be able to convert the entire vBulletin Application to Oracle and Stored Procedures (big performance enhancement) in about 30-45 days.

After conversion there would be little-to-no ACTUAL SQL in the product though because it is much more efficient to use stored procedures.

I personally would be willing to undertake this project for proper compensation.

The same goes for anyone wishing to run vBulletin under Sybase or SQL Server.

If anyone is interested you can contact me through the email form.