This really depends on the VPN server.
For the case of vpn3000, you can use the vpn client and make use of TCP transparency feature to vpn through the pix.
If the vpn server is a PIX or router, this Nat tranparency is not yet supported by the code.
If the VPN is PPTP, it would not work through the PIX using PAT, you need a static nat for every host.
Regards,