Md. Monir Hossain
regarding the VPN issues.
you need to know which ports are block by the firewall.
are you using VPN Gateway, if so there is every minimal interconnection problem. if you can ping each others IP(main office and branch office) with very low latency.
but if your using VPN client base, well its a big problem because VPN client only permits the computer who dial the VPN connection.