I would not rely on a waiver. Maybe the best outcome is that they have an abatement contractor remove the insulation without your involvement, and then you proceed with testing and improvements. Maybe refer the owner to this: http://www.epa.gov/asbestos/pubs/verm.html
Andrew, received the following from you but don't see where a reply can be posted:
I have a customer with vermiculite in the attic. I don't want the liability of running a Blower Door test, and made the case to the customer why it's not a good idea to test. However, he is willing to sign a waiver stating that he would not hold me be liable. Question: does anyone run the Blower Door (or duct blaster) with known vermiculite or asbestos in the home? Is the waiver a good idea?
I would be cautious of relying on any type of waiver. The law takes a dim view of a professional who tells the consumer that something is dangerous but that they can sign a waiver. Not worth the risk IMO, but I'm not a lawyer and don't play one on TV.
What about pressurizing the house with the blower door instead of depressurizing?