[RickRick]

The waterproof part is … moisture doesn’t affect the tile. Typical cheap laminate flooring has mdf base, it cannot tolerate any moisture. The lvt doesn’t claim to give a waterproof membrane, just that it is waterproof.

I was (am, slightly) concerned that water sitting on the LVP would seep between the seams and wet the wood flooring, what is it plywood? And then it wouldn't dry easily at all. Then over a few years there'd be damage to the subfloor or maybe mold. In a kitchen or mudroom I'd not think this would be any issue, but for a bathroom used by the kids in the house I can see some water laying there. I mean, the kids are pretty good about cleaning up but you know how somehow water from the shower seems to pool onto the floor, or else the kids get out of the shower before really drying off and they create the puddle.

A tile floor with a water barrier underlayment should be better, but not because of the tile - only because of the water barrier underlayment. But even that depends on which underlayment and how it's installed.

There are several tile "uncoupling membranes", and those aren't designed for waterproofing but to protect against tile and grout cracking. Of the membranes that I'm aware of, only one is waterproof as well, and that's Ditra.

I suppose one option is to install Ditra and then your LVT on top of it if you really want to be sure. But as chandler said, the better quality ones that claim to be waterproof actually are. You could install Ditra and then fill in the cavities with thinset, then LVT, but that seems like overkill. Maybe the RedGard idea would work fine.

I think LVT can at times be harder to install than people think, and tile can be easier (but you do probably need a tile saw, at least a cheap one). With some leveling clips, it isn't that hard.

We had that underlayment installed in a mudroom/laundry room.

I do see companies like Coretec advertise the LVP as being waterproof and for use in bathrooms. I also know someone promoted Quest pipe, which turns out to disintegrate from the inside when exposes to chlorine. :/