A VPN tunnel would suggest you can enable traffic on any port you wish without additional encryption.
So, assuming you have enough bandwidth, you can treat the servers as if they are on the same local network. The bandwidth is the big deal when dealing with a VPN, so you will have to determine which type of replication and latency allowed based on the amount of bandwidth available and the amount of data you need to synchronize (and of course if the replication is bi-directional).