how can i allow the traffic from (servers) internal vlan to the external vlan ?

SNAT is not an option if you want the server IP to remain the same. One possibility would be to use a L3 Forwarding virtual server. You can be very specific (down to IP or even IP:port) with the destination, or very open (0.0.0.0/0:*) if you need something generic.

Article K7595 provides an overview of L3 forwarding VS.