How to forward a port in UBUNTU on an internal VPN client?

Question

VPN

There is an ubuntu server (external IP xxx.xxx.xxx.xxx), a PPTP VPN server is raised on it, a VPN client is connected to it, its IP is yyy.yyy.yyy.yyy.
The task is to forward port 80 from the external IP to the internal IP of the VPN client.
Editing iptables:
iptables -t nat -A PREROUTING -p tcp -d xxx.xxx.xxx.xxx --dport 80 -j DNAT --to-destination yyy.yyy.yyy.yyy:80
iptables -t nat -A POSTROUTING -p tcp --dst yyy.yyy.yyy.yyy --dport 80 -j SNAT --to-source xxx.xxx.xxx.xxx
Not resolved, HELP......