The public SFTP server must be running an OpenVPN instance that is configured to be part of that same virtual network. If this is not the case, OpenVPN cannot help you.
If this is the case, the SFTP server will be listening on its OpenVPN IP address. It's likely the first address in the subnet. When you start up your OpenVPN instance, you should be able to ping that address.
For example, on a server I have OpenVPN set up as a server. It is set to accept incoming connections on its public IP, port 22220, and to create a virtual network using the private IP address range 192.168.64.1 to 192.168.64.126.
On the client end, OpenVPN is set up to connect to my server's public IP address, port 22220. I start my OpenVPN client, it connects to the OpenVPN server on its IP, port 22220, and a tunnel is established.
At that point I can type
ping 192.168.64.1 and reach the server that OpenVPN is running on, and since this is on the virtual network I defined, it is for sure going through the OpenVPN tunnel.
So if I had an FTP server running on that system, I'd tell it to listen on 192.168.64.1. You could then point your FTP client to 192.168.64.1 and connect, through the tunnel.