I had a project a few months ago dealing with a few different databases, some outside oauth calls, and absorption of keys from the outside calls. We ran into an idea to use the current vbulletin database as a basis for the entire website. We were capable of using several servers to keep things from breaking, but were a bit wary about putting all the content on one database.

My question is, with a full featured, database driven website using outside calls, custom database structures, and an already built database system (be it vbulletin, wordpress, or something of the likes), would it be best to keep things separated, or to absorb the current database structures, and extend the tables with new column variables?

We ended up doing 2 separate databases, one for the vbulletin (just in case there was too much activity on the website) and one for the overall website (smaller database calls, and a lot more caching involved). Ended up working out great, and we actually did not have to worry about any crashes on either database at the end of the day.