Thanks for all your responses. I think I've worked out the issue.
I am using a NetEA experimental list. I use the Saranes Expanse list which allows me to use Marines and Guard, mixed.
To build this I've used the online army creator:
http://traitor-legion.appspot.com/choos ... sade_NETEAThis allows me, when selecting Imperial Crusade Tank Company, to change all of the Russ' to Vanquishers, for no cost.
After your responses I've double checked the PDF (NetEA-ArmyCompendiumS1.2) and it clearly says 'One Leman Russ may be upgraded to a Leman Russ Vanquisher +25 points'.
It's an error on the Force Creator, and has screwed up my list slightly. I'm glad I checked this before fielding it.
Sadly I'm guessing the PDF is correct and will now be fielding 10 Russ' rather than 10 Vanquishers (boo). I've posted this issue on the ArmyForge Thread.
Thanks for all your help here.