A shared vacation? Yikes. I'd be expecting problems and try to head them off.
I don't think my metamour would be at all willing to let her husband sleep with me every night and I would probably push him to sleep with her at least half the time. At the VERY least I'd talk to her about what she thought that she needed, and let her know that if she changed her mind and was uncomfortable to please let me know. I'd also tell her that her bluntness was welcome, and that I trusted her to TELL me if she had a problem....
But, honestly? I don't foresee a joint vacation in our future. Maybe several years down the line? Maybe? As well as we get along(we get along fine!), that still seems like a distant possibility, something along the lines of winning the lottery.