In our new building I want to install vinyl flooring 5 mm thick, now there is a screed in each room.
Do they need to be sealed? If so, what material should I use? This is a concrete screed.
We are talking about a minimal small cut in the floor. With tile or parquet/laminate, you wouldn't have to cover it at all. For very thin vinyl it makes sense, but you don't need extreme stability with such a small hole. The most important thing is to have a smooth surface at the end. Tile glue, self-mixing concrete, etc. are suitable in this regard.
5mm doesn't sound like vinyl. It has to be perfectly level. Either way, you should use a leveling compound there, I would fill the holes with it as well.
The leveling compound is not needed to adjust the height, but to get a really clean flat surface. If you put vinyl flooring on, you will see every speck of dust or even the tiniest unevenness that remains even with a sanded screed later on.
You can buy small containers of cementitious repair compound at the hardware store. There may still be a small amount of putty left in the layer of flooring you trust. Mix this with a little water so that the mortar can still be well distributed, and then squeeze some of the mortar into the hole and sharply pull the remainder across the surface of the screed.
If there are "bumps": sand off with coarse sandpaper, vacuum.