CentOS7 as Router using PPTP as WAN?

[and1049]

Hi gents,

I've got a setup in the lab here to test out CentOS7 and we're wanting to route traffic on the lab LAN through the CentOS7 machine using a PPTP VPN as the only WAN.
VPN Server on the Internet <---- ppp0 on CentOS box <---- eth1 <---- lab network

I can make the VPN work and I can make the lab network talk to the CentOS box (10.99.99.1) but no guide I follow or anything I try gets the two connected. Any help would be MUCH appreciated.

Thanks!