Is this small slope a problem installing vinyl plank?
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: