Yes, you can. That's how I have mine set up anyway. My vB3 and PPC are on the same mySQL database. I have it so there's no prefix for vB3 and PPC is using a prefix of photopost_ (off the top of my head I think that's it) though you can use whatever you like for a prefix so long as they are different.
The PPC manual says:
2.) Database Configuration
If you're using PhotoPost Classifieds with an existing forum, you can either use the MySQL database that you're presently using for your forum for both PhotoPost Classifieds and your forum, or you can create a separate database for PhotoPost Classifieds in either case,
your forum user database will still be used by PhotoPost Classifieds to allow one centralized registration and login system). If you choose to, or if you are using PhotoPost Classifieds by itself, create a MySQL database for PhotoPost Classifieds and record the database name and the username and password needed to access it. Be sure to consult http://www.mysql.com
and your web host if you have questions about installing MySQL or setting up a MySQL database.
If you plan to use PhotoPost Classifieds with an existing forum, we recommend that you backup your database before proceeding. PhotoPost Classifieds is designed to work with the "stock"/unaltered versions of the various supported forums, not hacked versions. Most hacks should not affect PhotoPost Classifieds use of your forum database tables, but backing up is the safest way to go.