Should I add batt insulation to concrete basement walls?
I finished a basement in MN and spent a whole lot of time researching it. I eventually settled upon a method using XPS, steel studs, and paperless sheetrock as recommended by Building Sciences corporation and the US Dept. of Energy. I detailed the specifics on the answer to this question here:

https://diy.stackexchange.com/a/8644/1209

As for your questions:


I strongly recommend not ever using a vapor barrier in a basement. Read through my answer above for some more details. But, in summary: XPS acts a moisture retarder, but will still allow trapped moisture to dry to one side or the other. And since a basement can have moisture on either side, it's good to let it breath. When I finished my basement in MN, this was the biggest challenge I had with the St. Paul building inspectors. They insisted on vapor barriers, though invariably, in every basement I visited in MN with a VB, it smelled musty. I eventually did enough research to show them that the times have changed and VBs are not recommended that much anymore in basements by modern builders. 
adding the fiberglass likely wouldn't hurt anything...it's more insulation. But it may not add a whole lot, either, if the XPS is already sufficiently thick enough. So, the question is how thick is the current XPS? If it's providing a good r-value already, you might be good to go there. Save the time/effort and just stud out from there (using Metal studs, IMHO). 


Good luck!