Mark,
Normally you do not need to have inspection over VPN:
1) Typically you do not have NAT over VPN (typically!)
2) All the traffic is allowed over VPN by default (there is a sysopt for VPN traffic to ignore ACLs)
Now if it really is a problem of inspection, it's hard to say.
Did you try skinny instead of SIP? (Just out of curiosity).
Marcin