Talk to references; be wary if they are unavailable.
Make sure the vendor can articulate your business objectives and strategies and is interested in how you can integrate a site with your overall marketing plan.
Determine the stability of the developer. Will it be able to respond to your needs quickly with customer support?
Make time for a live demonstration, and ask question to understand why a particular technology or design was created; was it appropriate for the targeted audience?
Determine whether the developer can provide detailed Web site access reports. If collecting data about visitors is important to you, make sure your vendor is set up to do this.