After emailling their customer support on numerous occasions theyve now decided that the Huwai HG612 is casuing the problems.
They're talking out of their rear end, the HG612 will NOT be the cause of your issues. I'm using OpenVPN on a Netgear R9000 (DD-WRT) connected to a HG612 and it works like a dream. As Mr S has suggested, post your config details over on the DD-WRT forum and one of the nerds there should be able to help you further.
If you still don't get anywhere then perhaps its time to ditch IPVanish and join a high quality VPN service such as vpn.ac or airvpn.org.
TalkTalk Business 80/20
..(soon to be FluidOne FTTP 330/30)
Netgear R9000 X10 running OpenWRT
My Broadband Quality Monitor