Basically, you want to know why the company made the decision to go with the vendor. You might be surprised by what the reference says. You could also discover things that you didn't know the system could do. If the company has used the system for a while, it has probably seen everything. Find out how the vendor has addressed such issues.
Implementations can be nightmares.
You need to know what to expect. Ask how you could make the implementation smoother. If you have a hard start date, you need to know if you'll be able to make it. If so, ask to talk to references that used the same implementation team to find out if there were any glitches. This question could reveal to what extent the company is putting its future in the system.