Dealing With the "Current Vendor" Sales Objection

Here's a typical exchange between a prospect and a salesperson when this common sales objection is raised:

Prospect: "We're already working with xyz firm."
Seller: "Oh. How are they doing for you?"  
Prospect: "They're okay."
Seller: "What will it take to get you to switch?"
Prospect: "We're not interested." Click!

Clearly, that's not working.