If I were in your shoes I’d sit down with the son and ask what he’d like, and try and explain why I’m uncomfortable being around his mother and her new fling. Then in the conversation figure out if his desire is greater than my hesitations, come to an understanding on what we feel the vacation would look like.
Would it be all 4 just… Hanging out? Then probably not. That’s the most awkward. Or would it be that there are things to do (planned) that could pair off and have some time with son, while his mother and fling does whatever they do? As long as those times are more frequent, and son likes the idea, sure I’d probably accept.
You can self host Vaultwarden, which is essentially self managed Bitwarden.
And the feature can be setup fairly easily.