I barely have room on my dining room table for five heroes + villain + environment. I'm tempted to try 3v3 Vengeance for the sake of giving it a go but I am not sure the space is really there for it.
I've also heard that it's harder than the more normal mode of play
Roughly double the space for each character as if you're playing standard. One player would get a hero and a Vengeance villain. And you'd still need room for an Environment.
I tend to play Venegance games on the floor because of the space needed. I line up villains and heroes alternately below each other with the enviroment at the bottom.
This helps me keep track of whos turn is next and makes for logical play.
Also, I play solo a lot so if you play in a group this might not be the best way to do things.
I play all of my games on a small coffee table the characters are lined up where the villains are on the other side but in between the heros who are in the front. The environment just gets the leftside.
You can also use the image on the villain cards as space for their ongoings or monks after all they only flip when uncapped wich also means they have no cards in play at that point.