The transition height, if a major concern, would have been dealt with at the design phase. The designer would have calculated/estimated the height variance and would have specified the subfloor under the LVP be RAISED by 1/2" or 5/8"...whatever would be closest to the actual height of the cement tile. The outside flooring needed to be raised to meet the cement tile INSIDE the bathroom.

It sounds 'bass akwards' (raising 3000sf of flooring JUST to get a 100sf bathroom floor the same height) but it would have been the only way to do this. If this is on a slab you *MIGHT be able to jack out 1/2" worth of concrete (yep...remove material underneath the tile so that the floors sit flush) but that is a drastic measure which isn't something you want to look at (you NEVER know what you will find when you start breaking out concrete....there is nothing fun about hitting a water line).

At this point, you have no other option other than an uneven transition (slopes down to the LVP). The one thing I would suggest, if this is merely an aesthetic choice (the LOOK of cement tile is what you are after...not the material itself), is to look at "cement look" vinyl. There are some products out there that mimic the look of cement encaustic tile (is that the type of cement you were hoping for) or even 'cement look' vinyl (looks like grey slab of concrete).

If you work with vinyl on BOTH sides of the threshold, you can have your cake and eat it too.