Like the main campaign, the Vestal struggles to deal sufficient damage and is used to keep the party healthy. It is even easier to maintain party health in the Circus with her buffed healing skills, but Stress, DoT stacking, and Stuns will overwhelm her capabilities. That said, she also has a niche as a backline Stunner, with her Dazzling Light letting her control the enemy party's actions when she doesn't need to heal.
With a Vestal, it's good to put her in rank 3 or 4, out of harm's way and out of range of many stunning skills. However, be aware that she will still be a priority target as one of the best healers in the game. Pair her up with a Man-at-Arms so that he can protect her from harm, but also be aware of anything that can bypass Guards if you choose this. Equip the Vestal with an Exotic Snuff to compensate for her low Stun and Move resistance, because getting her dragged to the front will disable your main healing skills. A close-range skill like Mace Bash is also a good emergency attack if you do find her in that position.
The Silver Syringe is a great Trinket to use to amplify her healing ability, which can be replaced with the Idol of Purity once unlocked. The Brass Knuckles can augment her Stun rate of Dazzling Light, and can be replaced by the Tome of Fury which offers even more benefits to that skill. Illumination can become a close-range emergency attack that benefits off that Trinket too.
As the best healer in the game who can keep the party together and bring them back from Death's Door repeatedly, the Vestal is a priority target. Pulling her up to position 2 or 1 will disable her healing skills, leaving her vulnerable to more Stuns and the rest of your assault. Pay attention to anyone who can guard her, though, as they can become an even bigger priority target over the Vestal. You might also want to consider Dazing the Vestal so that you can secure a Deathblow before she can heal them to safety.
As the Vestal has little in the way of Stress prevention, driving her into an Affliction can work to your favor. A Vestal passing her turn at an inopportune moment gives you the opening to pick off anyone at Death's Door.