Let me make sure I understand. You and S broke up largely because of his overly involved, controlling mother.
After the break up, you dated V for a week. At the end of that week, you had sexual contact of some sort with S. You told V about this; he made you chose and you chose S. Then S just wanted a friends with benefits situation but later changed his mind and now S and you are back together as boyfriend/girlfriend.
Did you and V explicitly agree to be monogamous with each other? Or is that just understood to be what people do in your area? If you didn't agree to be monogamous, I don't see how sexual contact with S is cheating. I would call that 'dating' actually. If you haven't made committments to anyone, then playing the field is totally ok, and not cheating. Of course if you and V agreed to see no one else (after a week!), then, yes, you did cheat and that is unfortunate.
Have you made explicit agreement or committment to be monogamous with S? Or is this all unsaid or 'understood' as what 'everyone' does in relationships? Because nothing will fuck up relationships like unsaid expectations.
If I understand your situation right (and I'm not sure of that) it seems like you should talk with S first that you may want to explore ethical non-monogamy and/or polyamory. I would talk with S first because he is your boyfriend at the moment and so should hear this first from you. If S is willing to explore, then I would approach V about it and see what he thinks.
Of course, understand that you decide if you want to be ethically non-monogamous or polyamorous, not S or V or anyone else. Of course, they may decide not to go this journey with you. That is a risk. But it is your decision.