Always use quelling if you have it!

Just because a good tank CAN keep aggro from you without it, it's gonna be a dps loss overall if they do. I have real trouble keeping aggro off our smn even when he uses quelling sometimes on bosses, and we're about equally geared. Every time I have to use halone outside of my initial hate combo I cry a little.

Although in your case the pld was just really bad. Marking in the Vault... The only use I can see for that is to mark the one or two enemies per pack that are toughter than the rest, maybe? Even then... I mean, blm can AoE from sastasha. You should be used to that as a tank by now. If you're using all your aggro management (in blm's case, that's just "quelling before opening eno burst") and the tank still loses aggro, yeah, that's on them.

Mind you, if you're NOT using quelling on your burst and you're riding the line of enmity, there's still stuff you can be doing to help. It's not like quelling reduces your dps (which I have encountered people who actually think that, so...)