Yes check out the 1820 club, they usually meet 1x a day with a youth counselor for an organized gathering. First night meeting is usually in Cove Cafe for coffee tastings--going the first night is important to meet others their age and to discuss the types of activities they'd want to do. On our 2018 cruise, they met for lunch 1 day, did crafts, had some sports thing, I think they had an ice cream social as well, and other activities. A lot of times the kids would arrange to get together at other times during the cruise without the counselor. My oldest didn't click with anyone so she just went to a couple of the meet ups and did some adult only activities with DH and I--she really enjoyed some of the adult only entertainment. Yes, there are open house times in ALL the clubs at some point, so that's when they could visit Vibe, they just have to leave when the open house times end.
And at 18 they can go to all the adult spaces, including the fitness center/lounges--they just can't order alcohol. They should have their key card with them at all times as it identifies them as an 18-20 yr old (letter B vs A for adult and M for minor) and grants them access to all adult only areas.