every time a user log on, the VPN gets disconnected and the connection has to be

Hello Guys:

I have a partner with a Windows server 2012 R2 working as a RDS to give remote access to users. This server is on a remote location from the Datacenter and is connected via VPN (RAS-PPTP) as a domain member.

They need the VPN up all the time, but, every time a user log on, the VPN gets disconnected and the connection has to be reinstated.

They have checked and everything looks ok, Where could it be the problem?

Thanks.