VPN can be added via an IOS upgrade/change, however on the 3640 there is by default no hardware accelerated VPN. I think there is (was?) a VPN Accelerator NM module for the 3600 routers that handled the VPN encryption/decryption. You can do this in software it just isn't as efficient.
All the new ISR routers have VPN acceleration hardware built-in (I think, certainly my 877 has).
HTH
Andy