I honestly had to laugh at the idea that this has to be explained, as in (and I'm not trying to mock you, I think it's about life experience as a parent and FOO issues that pretty much everyone has) have you ever been on a vacation with your parents?

LT has already explained some of the specifics, but I would say that times where I, H, and the child have vacationed with either my or his parents-- which we did fairly often, it was always quite stressful. Maybe some parents are different WRT to their grandchildren and the parenting by their own children, but a lot of judgement and explanation of how we do things (no, we don't think he needs a spanking), trying to negotiate for things that will benefit our kid and for what he needs amongst the "family time", and about a thousand other ways where even in a pretty civilized FOO the back and forth between what to do and how to do it, when to have dinner, where, etc.

Vacation with the FOO is always difficult. Lots of times there are benefits especially because you want to facilitate the grandparent relationships and they can be useful like paying for expensive stuff and babysitting. But being around them and sharing some kind of space flags the freak in them, or I could say it did for me. My mother is infinitely more controlling on vacation and ultra flexible when she comes and stays with us.

But these multi generational vacations are like a giant vortex for co dependent behaviors, because now you don't just have to please your partner and your child but these other people as well.

Every vacation with the FOO was a vacation I needed a vacation after.

Anyway, not an intentional hijack of LT's family situation, but I'd bet at least a dollar that some of my issues are similar to hers.