In the quest "Family Pride" you have to solve the tension between two noble families - Bardatto and Valera. This quest is also apparently important for one of your companions.
This quest also relatest to two other quests: in "A Sinking Feeling" you discover that Valera are planning to break into Bardatto's vault and Valera are asking you to do a small, seemingly unrelated favour in "A Bigger Fish".
Anyway, "Family Pride" popped up quite early on my quest list but was marked as too high level, while the two side quests were much easier, so I've done them quickly. Upon completing them, I've heard from the head of both families, that the heist might still take place "soon" - I was asked to help both sides, but again, the quest was still marked was too difficult, so I decided to not commit yet.
When I came back to it with my companion Pallegina (who wants a peaceful solution), I could talk to the head of Bardatto family and propose a meeting, but when I've talked to Valera all I could do is either agree to save someone from this failed heist or attack them. When I've agreed, Bardatto became almost immediately hostile and had to kill them all, which made Pallegina leave permanently (it happens only if she is in the group during that fight).
But from the conversation in the vault with the guards and the would-be-thief, who thanked me for saving him (her?) and suggested escaping via the window to avoid more fight upstairs (but I've already had to kill everyone upstairs to get to him!) that you could somehow sneak to the vault, save the thief, avoid the slaughter and maybe still get the peaceful outcome.
So, is it possible to negotiate the truce AFTER the heist has started? Maybe with an insane high sneak to get past the guards to the vault?