You write that if you don't talk with either of them soon, you may end up in bed with V.
Since you are now "officially" the GF of S, by having sex with V WITHOUT talking about it, you would "officially" be cheating on S. Are you OK with that? Would you be OK with S cheating on you? (I don't mean sleeping with somebody else, I mean lying about it [or hiding it, which is another way of lying].)
Remember that you also wrote "[...] I cheated with S. ( I am very not proud of what I did
)" Do you really want to repeat that?
In my opinion, honesty is one of the most important factors in a healthy relationship. You will end up hurting yourself and S if you cheat on him. You will be bothered by having this terrible secret that you have to keep hidden. And he might break off with you if he ever DOES find out. If V knows that you are with S, don't you think that he might reveal the secret some day? (If, for example, you and V have a fight [or V and S have a fight] and V wants some cheap revenge?)
And you wrote that V and S are friends. How would you feel if your and V's cheating on S damaged (maybe destroyed) THEIR friendship?
What are your other options?
a) Decide not to do anything about it and stay monogamous with S. And friends with both of them.
In this case you might continue to fantacise about V and even feel frustrated, but you can learn to live with that.
b) Talk with both of them honestly about your feelings. Don't say "This is what I'm going to do!" Say "This is how I feel. Can you imagine something like that?" If they can't accept it, then a cheating relationship wouldn't have worked anyway. If they can, you've all got some work to do... to keep open and honest with each other. It's not easy, but I think that it's worth putting in that extra work.
If you talk honestly and they both reject your wish for polyamory, you then have more options. You can learn to live with their decision, you can hope to help them change their minds in the future, or you might decide to look for partners who are open to polyamory.
It's tricky convincing a boyfriend that your wish for polyamory isn't a sign of his not being able to satisfy you. But if he earlier prefered the option of "friends with benefits", it suggests to me that he maybe isn't TOO possessive, and might be open to polyamory. It's worth the risk. Be delicate, but be hopeful.
And act in a way that you can be proud of yourself.