How to fill gaps in floor after wall removal
Wood would be best for the here and now and to be gone for later. IF, you'll be removing the existing flooring for the LVT to go in.

Let the LVT installers simply lift out your wood and fill the gap later with Floor Leveler or other cementitious product. It's very difficult to chip stuff away to perfection, but very easy to put it in level with a new permanent elevation.