When VPN tunnel fails between primary Domain Controller and secondary DC - What happens?

I'm a little unclear as to what happens when a VPN tunnel fails between a primary and secondary Domain controller for an extended period of time. Here is a basic rundown of our environment. We have in our primary location the DC1 we recently moved out customer facing sites and DataBase (the DB is the only server on AD, the rest are DMZ) over to a data center and stood up DC5 (we have other domain controllers in various locations) we have a VPN tunnel between the two locations. I understand that systems will continue to function as normal for short disruptions to the VPN. My question is what if that tunnel is down for a couple of days or even a week. does DC5 continue to work with out DC1? is countdown to failure at some point?

My background is software development, our IT guy says after "some undetermined time" sites would fail to work if the VPN is not active. I am questioning that statement.

Thanks