A melee dot tree, which is the vengeance spec, doesn't seem very useful since applying all of your dots to a target requires staying on them, building up sunder stacks, and not getting peeled at all because if your stacks fall off you have to start all over. Not your fault of course, now the immortal spec actually does manage to hit pretty hard along with several cc's and defensive cd's, I've crit pretty high with my crushing blow and with that and one sunder I have 5 stacks in two gcd's. But if you are insistent it seems like your best bet is to slow and snare targets around you while spreading dots as best you can. Immortal might suit you better with it's variety of damage, defense, and utility, have you tried Rage yet though? I haven't, just wondering if it's any good. As for your targeting, granted tab targeting in this game is a little wonky, but aside from paying better attention I don't know what to say. Edit: Don't discount peeling and snaring. The low cost aoe slow is awesome, sometimes just making the other team miserable is good enough.