For our OpenVZ Services, PPP/TUN is automatically enabled, and hourly checks are performed on each container's configuration to ensure it is working. All that is required is that you reboot your container at least one time after creation; or to be 100% sure of functionality, any time after XX:00h.
You can verify TUN/PPP functionality with the following commands:
cat /dev/ppp > cat: /dev/ppp: No such device or address
cat /dev/net/tun > cat: /dev/net/tun: File descriptor in bad state
Should either command return a different error, please wait an hour (after your VPS has been created) for the automated fix to resolve the issue.
IP Masquerading is not available on BuyVM/OpenVZ, however you can work around this limitation with the following command:
iptables -t nat -A POSTROUTING ! -o <tun|ppp>+ -j SNAT --to-source YOURVPSIP
Use tun for openvpn and ppp for PPTPd.