Finish vinyl plank flooring (subfloor step)
This is probably something you should have thought about before placing the new flooring :)

However you should probably put a door sill in.
As for material there are various options usually stone, wood or metal.
You'll have to choose what best suits your home.
If you want the drop to be gradual you should get an angled sill.
Most likely you will end up having to cut into the newly laid floor a bit to accommodate for the door sill.