What are your top IT vendor management best practices?

Communicate, communicate, communicate.

Communicate your expectations with them.
Communicate when those expectatons are not met
Communicate when they are met. 

Working with a vendor is a relationship - and like any relationship, it will go bad if you aren't communicating. 

90% of the issues I see between clients and their vendors come down to not doing this.

Like any relationship, vendor management goes by good relationship building. Yeah, its not a documented best practice, but understanding and fostering good relationship with vendor goes a long way.
Look at how you can partner, than treating them a vendor