I may be mistaken but is there not 3 manufacturers that produce a Tahiti based 7870?
All I can come up with is the Sapphire 7870 XT and the PowerColor 7870 LE.
Alright Club3D is the other one I was trying to think of. I have never heard of VTX though, do they have them in America? I will check since the website you posted is from the UK