If you haven't tanked this yourself, I'd recommend you give it a try. Pulling adds to the front isn't as easy as folks seem to believe.
To start with, whenever odds pop, they pop far apart from one another. Snagging them all with Weapon Throw or Shield Lob takes far too long, so usually tanks will focus on grabbing the big add and wait for the little adds to gather together, usually on the healer. Of course, once they're gathered on the healer, it may take a while to pull them off with Flash or Overwhelm. If your healer is smart, your healer will be pulling all these adds to the front of Vishap. Very often, though, your healer is not smart, and will be lingering behind, spamming cures on themself, or worse running around in a panic trying not to be hit by all the little adds. All of this can add time to how long it takes to gather the adds at the front of Vishap.
A lot of folks have the DPS focus down the big adds, since it takes the cannons so long to kill them and the big adds can do so much harm while they're still alive. This isn't a bad strategy - but the big adds should STILL be pulled to the front of Vishap along with the little adds. If the DPS focus it down and don't move it, the tank may waste time trying to move it to the front of Vishap where it should be.
To sum up, getting the adds to the front of Vishap is a TEAM responsibility, not just the TANK'S responsibility. The tank's job is to hold them there - GETTING them there is on everyone, particularly on the healer since most of the adds will beeline for the healer.
If you, or anyone in your group, has an add beating on them and is wondering why the tank isn't pulling them off, it's because the tank is preoccupied at the center of a swarm of adds at the front of Vishap. Running back to get the one beating on you means pulling that cloud of adds out of cannon range. If you have an add on you, bring it to the tank. Don't wait for the tank to come get it off you.