This depends on a few things - what type of VPN does the asterisk support?

If you have something like this:

CME--ASA--VPN--(Mystery Device, or another ASA)---Asterisk

Then there is absolutely no problem. Both the CME and Asterisk would see it as a simple layer 3 network and not care.

The only problem you would run into is interoperability support for VPN on asterisk if you want the VPN to be point to point between the CME and asterisk. Then I don't have a clue :)

-nick