I use this rules:
iptables -t nat -A PREROUTING -i eth0 -p tcp --dport 80 -j DNAT --to 10.8.0.26:80
iptables -A FORWARD -i eth0 -o tun0 -s 10.8.0.26 -j ACCEPT
iptables -t nat -A POSTROUTING -s 10.8.0.26 -j MASQUERADE


this is my rules:
# iptables -t nat -vnL
Chain PREROUTING (policy ACCEPT 1 packets, 40 bytes)
 pkts bytes target     prot opt in     out     source               destination
    3   192 DNAT       tcp  --  eth0   *       0.0.0.0/0            0.0.0.0/0            tcp dpt:80 to:10.8.0.26:80

Chain INPUT (policy ACCEPT 1 packets, 40 bytes)
 pkts bytes target     prot opt in     out     source               destination

Chain OUTPUT (policy ACCEPT 0 packets, 0 bytes)
 pkts bytes target     prot opt in     out     source               destination

Chain POSTROUTING (policy ACCEPT 3 packets, 192 bytes)
 pkts bytes target     prot opt in     out     source               destination
    0     0 MASQUERADE  all  --  *      *       10.8.0.26            0.0.0.0/0