the spawns do not attack your pet
Just went to try to duo Vrtra and the adds DO ATTACK YOUR PET. EDIT: You can also be drawn-in. I did nothing but sic my Dipper on it, from max distance and about 5 minutes later I was drawn-in. I think you got extremely damn lucky on the not drawn in part.
Wierd, I've solo'd Vrtra a good 6-7's now and not once experienced this. My diagnostic: It's something to do w/ Horrid Roar (The Grand wyrms have an enhanced version, wipin' all buffs and VE) although you've specified to only making Dipperyuly engage Vrtra and rewarding Dipper.. generating no legitimate hate.
I'd say it's maybe a glitch and you should just retry, as I said this hasn't being occuring to me and others (or has it?) so I don't really know what it is, but that's what I'd say it is.
As for pets joining the fight, they wont unless the following:
You are close to Vrtra without sneak
You approach Vrtra in yellow HP for Whatever means
A player outside the party does the above.
Either the adds are glitched (likely scenario) or their game mechanics are to engage the fight with Vrtra on
players, or engage Vrtra's target when
players approach the adds.
This has been happening on other such BST mobs, though. A quick example: Archamoth