route traffic to one of the rb750 port

Hello,
i have a lan connection in our office if somebody want use internet he/she should connect to our external vpn server so i can connect to our vpn extenral server with rb750 but i do not know how route my traffic to my vpn connection and send the output to port 3 port 2 is for our lan connection and our DHCP server,
can sombody help me about this?
thanks,