So, basically, I'm just starting out with RMXP. After reading a hell of a lot of threads, I've concluded that XP has a better mapping system than VX and better (in my opinion: I've seen the VX graphics and I don't really like them) graphics, while VX has better scripting, event and database systems. I'm wondering whether I could use XP for mapping, and VX for eventing, etc. and whether there would be any problems, like not being able to use the VX save with XP in case I wanted to change things.

Unless you're really knowledgeable with both engines, and with scripting as well, no. And even in this case, it's roughly doubling your work. Just choose the engine you are the most comfortable with.

You may want to take a look at this:

http://wiki.pockethouse.com/index.php?titl...vs_RPG_Maker_VX

And like Kread said, stick with one or the other. They're both usable if you're willing to give it a chance.