Hi you could do that by DNAT in iptable on the PREROUTING chain
Below example will redirect http traffic from the range of ips specified ("start-ip" to "end-ip") to the destination ip and port specified(gatewayip:vpnportnumber).
iptables -A PREROUTING -t nat -m iprange --src-range "start-ip"-"end-ip" -i "interface" -p tcp -m tcp --dport 80 -j DNAT --to-destination "gatewayip:vpnportnumber"