I'm afraid I cannot answer your questions as I don't know what kit supports which features and I know nothing about Hamacti. I am only an amateur dabbler.

If you're setting up a VPN behind another router you start getting other problems, but it can work. I have no idea if VPN's from DrayTek kit behind another router will work. They might but there is no guarantee. Possibly a better option would be a dedicated VPN server. I presume they exist, but I'd have a go at building my own server (any old PC would do) and running Linux on it with Openswan. I have never set up Openswan in this mode so can give no guidance and I only dabble with Linux.

Also, to get a VPN to work from behind the Cisco, you will probably need access to the config on the Cisco. Can you get that?

No I don't think you can we tried something like this a while ago with no success! Our workaround was to install a 2nd DSL line in at Head Office and install a DrayTek on that to provide the Tunnel