So recently, I've been playing Cellblock on my account and I'm still only at tourny 2. TNT has stated in a NT editorial that the Cellblock Vexed Avatar was not awarded randomly, but rather awarded after some complicated/simple formula involving wins/loses ratio or whatnot. From the looks of it, people got it around tourny 7+ and most have had empty inventories (or so I've heard).
Post here your stats (if you remember) of your cellblock game and any available info at the time the avatar was awarded to you (like time in NST, inventory, pet, neopoints on hand)
My friend has a theory that you lose 2+ matches with Vex at tourny 7+ and then win that "level."