To answer the direct question – no, I don’t think I could vacation with my husband’s ex. (I’m the stepmom).
We have a very amicable relationship, all things considered – and I’ve co-parented for over 15 years..and cannot foresee a situation, aside from my beautiful step-daughter choosing to get married in a far away place, where we would all vacation or travel someplace together.
That said, we do all attend every school event, and we share a holiday get-together each winter near Christmas with his ex and her extended family. We’ve done this for years – and it’s a bit awkward (more so for my husband I think, oddly enough) but do-able.
When SD was younger I think it was more enjoyable for her to have all the people who love her all together (e.g. at birthdays and holidays). Now that she is a teenager and well aware of her mother’s true opinion of me (and vice versa) and can sense the tension that exists even when we’re all being very nice & getting along very civilly… she’s not really all that interested in having both halves of her family together all that much. Granted, in our case SD does not remember a time when her parents were ever together because they split up & divorced before she was 1.
So.. I’d say this depends on the age of the kids, whether there is any chance for conflict that will disrupt the holiday (hey, travel can be stressful enough…why add to it with tension w/ the ex?)...and whether the kids are pushing for this arrangement or not.