Was it a PUG? I have had endless issues with TankaDins in the past(PUG only), and in that group setup I think I may know why the VW was chosen. This is not a slam on any player, but the bottom line is Tankadins are not the best tanks avaiable. Sure when everybody knows each other, and nobody is in a hurry it works out just fine, yet if you look closely at that lineup you may see why the VW was chosen. A hunter can shed all the thret in the world ever 30 seconds with Fein Death, a Rogue can Feint or vanish, a priest can fade, and you want aggro, so what is it that a lock can do when they get aggro? Eat it thats what, Soulshatter dont realy count as aggro managment(in this context), not when every other source of DPS can litiraly dump aggro every 30 seconds or so without a large pricetag. For me, far too often the Pally uses conceration, the priest/druid/shaman/pally heals the Tankadin and it is all over, the Heal regesters more aggro than conceration and the mobs make a Bline for the priest, they pop Fade, but even in the best casethe situaion is not looking pretty for a "Ideal" start.
Secondly it could have been you were dealing with a Demo Lock who had the MD buff and he wanted to take 10% less damage, maybe they were out of shards and had to use the Abbsial shard to summon a VW(garbage logic the could have summoned a imp) and lastly, maybe the lock was kind of slow?
The only other thing i could immigine is that they were wanting to AoE and needed the sheild from a VW sac.