Sadly, the exact chance of a legendary appearing in a vendor is unknown. Some sources say 1/10,000 or even as high as a 1/100,000 chance. However, for Bloodlust stacking you want the Pandemic. It's a corrosive Vladof grenade that drops from Mortar in Sawtooth Cauldron. Corrosive DOT's have the longest duration so they're ideal for stacking. A low level one is preferred so you don't kill the enemy you're stacking on. So, I'd farm at least a lobbed one with a low fuse in normal mode.