This comment lists most of the questions I would ask. Couldn't agree more. Only advice I would add for the OP:

Check if your contract with the vendor includes any business continuity clauses. What happens to their solution, their code, and your data if they go out of business or get acquired (by someone else)?

One relatively common option is to have them put their code in escrow, and get access to it if certain events happen. You can probably negotiate this into your contract if you have enough leverage.

If their software is critical to your business, and you feel they are stagnant and underfunded, there may be some risk here you can address today - check with your lawyers.

(also, just noticed the username I'm replying to... hi Teren!)