That's crazy talk, Vette is perfect for the Marauder... but I can understand her not being very good with the Juggernaut.
From my time spent learning everything there is to know about tanking as a Vanguard, I've learned quite a lot about how to manage agro. That experience actually came in handy with my Silvana + Vette team up. Basically Vette and I shared the agro somewhat equally, though I purposefully got her to take a little more of the agro and at times die for me since it's over if I die... of course I used moves such as Force Choke to ensure that she was able to get some good DPS in as well. I'd also use moves such as Smash, Force Scream, Ravage, Crippling Slash, Deadly Throw, Obfuscate, Force Camouflage and Intimidating Roar... all of them to keep the enemy off balance in every single way possible to make sure that they weren't able to retaliate effectively enough against our combined onslaught. Disable Droid also helps quite a bit too
Like I keep saying, it ended up working amazingly well.
As for Quinn taking heat when we were playing, it was because he started healing Vette when she started taking heavy amounts of damage that got him to generate enough threat to be targeted... something which could happen equally as much if either one of us were taking large amounts of damage and he was healing us constantly.
It happens when with a full party group as well, it's up to the tank to manage it.