This is a floating floor that glues to itself flooring, the allure planks have a underlayment attached. It will be going over 40 year old wood planks that have had repairs in a ground level living room. Should I use a floor leveler or underlayment product over all floor 25' x 11', or just patch gaps and holes? Thanks.
Flooring – How should I prepare old subfloor for vinyl plank flooring
vinyl-flooring
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I'd question how waterproof you need this to be. Will there routinely be water sitting on the floor? If not, you may be over-engineering things.
That said, off the top of my head, one option for waterproofing the subfloor would be to paint on Redgard:
It's a 'paint on' waterproofing membrane that you'd typically use behind tiles on cement board and the like.
Yes, this will be a problem if done as is.
If you don't stabilize that flooring by leveling the floor beneath, you will find the floor will sink under your feet as you step on it in that point, and since there's a fixed threshold, you'll create a tripping hazard in the process.
The product you show - that's a bullnose. It's used on steps. It's not suited for your intended use and it won't work anyway because you're on concrete. They're meant to be nailed in place.
I suggest you get some patch leveling compound. It's a concrete product that's fairly thick. You apply it to the uneven areas of your floor with a nice wide trowel and use a long straight board to basically boost up the sinking area of the floor to create a nice level surface. That will keep the boards from sinking when stepped on and keep the top of the floor close to level with the marble threshold for a nice safe walking surface.
Get pre-mixed leveler. One container should be enough:
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How perfect do you want it to be / How much money and time are you willing to spend?
The vinyl allure product is at the cheaper end of vinyl products so my recommendation would be to forget about leveling and just make sure that the floor is not out 1/2" or more over 4'. So take a 4' straight edge or level and move it along the floor and make sure that any gap under the level is less than 1/2".
The vinyl flooring will conform to the rolling nature of the floor to a better degree than most other flooring types and if you don't mind it being a little bouncy in some places I suspect it isn't worth the effort to do anything more than ensure there are no squeaks, there are no large holes and the floor is within the 1/2" difference over 4'. The floor really just needs to be flat not level and pretty much the only case where I'd go to effort/money to flood a floor with self level is if that room receives no furniture and no area rugs. If you already know the areas where furniture will sit you can spend more time on the areas where someone is likely to walk and notice the bounce / dipping of the floor.