I'm not sure if I'm following you but here goes....
Yes you can allow multiple Vpn's into a Linux server.
http://poptop.lineo.com
The only snafu would be if you want multiple connections from the same ip address. So if the Client NAT's their connections to you it wouldn't work. Unless they have a Linux Firewall doing the NAT'ing. Then they could masq the VPN connections.
Anyway, as far as the data seperation goes you would just control the access to which ever data by permissions. You could keep the data in seperate directory's and apply permissions to allow only certain people in.
Hope that helps......
Cheers,
Scully