There is no support for any vector plug-ins in Xtreme.

There are two interfaces for bitmap plug-ins though.
1. The XPE plug-ins interface - it's new, but everyone can learn and make his plug-ins. It's advantage is that they will look like all XPE tools: enhance, redeye, bumpmap e.t.c. Those will be 100% Live Effects compatible and interactive.
2. The PS API - photoshop plug-ins interface. But you know about it, right?

That's it.