Specific need for tcp/ip routing with iptables

I would like to do the following thing:

• One server with public IP (say 1.2.3.4) and internal IP ( say 192.168.1.1)
• Multiple VPN routers connecting (IPSEC and OPENVPN) (port 500/1194/4500)
• Internal VPN Linux servers (192.168.1.2-> 192.168.1.100 for example)

I would like this :

• Connection from Specific IP (ie. 5.6.7.8) arrives on IPSEC and OPENVPN incoming portsand packets are redirected to -> 192.168.1.2 (to other ports maybe?)
• Connection from anotherIP (ie. 8.9.10.11) arrives on IPSEC and OPENVPN incoming ports and packets are redirected to -> 192.168.1.3 (to other ports maybe?)

and so on...

1. is it feasible ? (I have been searching but if I end up here, it's that I am out of options :) ).
2. if it is, how to do it with iptables ?Thanks a lotJG