Are you prepared to sign an escrow-agreement in case your company cannot support or maintain the product any more?

Escrow is the situation where the sources are stored in a safe at a neutral party in case the vendor goes chapter 11. You don't do sh*t with the sources as the programmers are not included. It's just a "feel good" measurement.

Don't trust anyone who doesn't want to corporate with an escrow.

Are you willing to participate in a technical audit on your product by a third party?

Just see how confident they are about there own stuff.

Are you prepared to provide us insight in your current order portfolio?

To avoid that they have more projects than they can handle just in the time frame you go live. It's very bad if you have to fight for your resources.

Could you provide us with a global project plan on how a typical implementation would look like?

Looks professional?

Could you provide us with an overview of the basic functionality of the product? What are the options? What enhance are planned for the near future (please indicate expected date)?

What the heck are we actually buying?

Could you please provide a description of the pricing of your product and the several parts of the product?

How are the prices constructed (e.g. site licenses, concurrent users)?