Few things to ask:
How actively does the VC (and the partner who is joining the board specifically) like to get involved and what sort of topics do they care about the most?
What happened when there was a disagreement between VC and founders? How was it resolved? (a wedge issue)
What areas has the VC helped the most? What areas do you feel like they could have helped more?
In general, if you're giving up a board seat you really want to make sure you're bringing on someone who is completely in line with your vision of the company for the next 5+ years and can help you get there, especially if there are gaps in founder's backgrounds.