Cait Sith/Reeve Cait Sith doesn't interact much at all with Vincent in Final Fantasy VII, but there is most definitely a future for these two. Reeve will be returning as the mastermind behind the moogle riding cat in Dirge of Cerberus as one of Vincent's allies. I'm curious to see how the two will work together and if they'll delve into their past ties with Shin-Ra at all. Plus, I'll get to see more Reeve, who I've liked since I saw him for the first time in FF7. I really do have a thing for Shin-Ra employees in suits.
Marlene Vincent had only heard of the little girl in FF7, but sometime after Sephiroth was defeated and the world was saved, he became a key figure in her life. Though they only had one small scene together in Advent Children, it was still made quite obvious that Marlene trusts Vincent and even treats him as though he were a close relative of hers. Vincent, who does not normally show affection for others, openly does so for Marlene by allowing her to hide in his cape when she becomes upset with Cloud. He even goes as far as to ask Tifa where a telephone shop is when Marlene becomes disappointed with him for not having a cell phone.
Barret I don't feel like I can fairly analyze this relationship because I almost never use Barret and nobody else seems to either so I can't research this, but here goes. I really don't see much of a friendship here, more of a comrade type of relationship. They really didn't have much to say with one another other than the fact that the world was in crisis and I really don't think they understood each other's personalities much. I thought it was hilarious though when Barret asked "That when you started sleepin' in a box?".
Red XIII Much like with Barret, Vincent doesn't seem to interact much with Red. If they ever did, they'd probably get along well. I could imagine these two having intellectual type conversations. That sounds kind of amusing when you picture it.
Sephiroth Vincent doesn't have a personal connection with Sephiroth, but since the former SOLDIER is the son of Lucrecia, he feels conflicted in whether he should help Cloud destroy him or let Sephiroth live and wreak havoc on the planet. In the end, he chooses the former, but I think he knows that it was in everyone's best interest to have it be that way, including Lucrecia's.