-A FORWARD -s 10.10.10.0/24 -m policy --dir in --pol ipsec --proto esp -m comment --comment "VPN forwarding" -j ACCEPT
-A FORWARD -d 10.10.10.0/24 -m policy --dir out --pol ipsec --proto esp -m comment --comment "VPN forwarding" -j ACCEPT

...
*nat
:POSTROUTING ACCEPT [0:0]
-A POSTROUTING -s 10.10.10.0/24 -o eth0 -m policy --dir out --pol ipsec -j ACCEPT
-A POSTROUTING -s 10.10.10.0/24 -o eth0 -j MASQUERADE

...
*mangle
:POSTROUTING ACCEPT [30:1812]
-A FORWARD -s 10.10.10.0/24 -o eth0 -p tcp -m policy --dir in --pol ipsec -m tcp --tcp-flags SYN,RST SYN -m tcpmss --mss 1361:1536 -j TCPMSS --set-mss 1360
-A FORWARD -s 10.10.10.0/24 -o eth0 -p tcp -m policy --dir in --pol ipsec -m tcp --tcp-flags SYN,RST SYN -m tcpmss --mss 1361:1536 -j TCPMSS --set-mss 1360