QuickBooks is not designed to operate over a VPN. It was designed strictly for a LAN, and running the data across a vpn will eventually damage the company file, possibly beyond the point of being repairable. The software has no capability to run the checks that a network/WAN software has, so it will assume any line noise is data, and must maintain a constant live connection to the data to operate.
Also, no matter how fast your vpn is, it is not as fast as a lan.
There are services that will host a Qb file and allow multiple users, you may want to look into one of those.

In line with the answer above, most routers will support VPN-pass-through - few support VPN clients or LAN-LAN VPN to the router - the reason being that this is typically quite an intensive function and consumer routers (which would categorize a large percentage of routers on the market) are not geared towards such activities.
So you need to decide what type of VPN you're using - PPTP or L2TP/IPSEC - and open the appropriate ports:
IPSEC - 500 L2TP - 1701 PPTP - 1723
As an example, Netgear routers will allow you to simply create a port-forward using the built-in services, however if your router does not have ready-made services, then you can often create a custom service using the above info.
Then just point the port to your the server that is running Quickbooks and you should theoretically be A for away.

First check to see if your brand of router will allow you to connect via VPN not all do..