If you are using Openvz VPS, then you go to your Client area > Services > Your Openvz VPS > Manage > Control Panel.

Click Port Forwarding and then click Add.

If you wish to set up SSH which originally port 22, then :

• Click Add, Select protocol TCP.
• Source/Domain IP - default.
• Source port ie. 3491 (select available).
• Destination IP (your vm local ip).
• Destination port 22.
• Click Add.

So you don't need to modify your VPS SSH config. You just need to forward the port and made modification from Port Forwarding rules.
Therefore, someone able to login to your SSH under port 3491.