Cutting an area of vinyl plank flooring that is already installed
Use a circular saw with a wood cutting blade set to the depth of the vinyl floor thickness.  Osc tools are invaluable for small jobs but they are slow and you'll burn through a lot of blades to cut something that size out.
I'd use some quick support rods that push down on 2x4s around the area where you are cutting to prevent any of the boards from moving when you are cutting.  Set the 2x4 and use it as a guide to cut a straight line in the boards.
Seems like a lot of work though.  I'd probably just put the wall on the floor.
LVT doesn't thermally expand to the same degree as laminate, wood or engineered hardwood.  I've installed kitchen cabinets on top of floating LVT without problem.  In your case you could secure the wall on the top and sides, to allow for expansion of the floor you could cut an oval hole in the lvt where the fastener holds the bottom plate to the concrete then if the floor wants to move under the wall it just has to overcome the friction of the bottom plate.  You could also put a low friction tape or platic surface on the bottom of the bottom plate which would allow the floor to move under the bottom plate with less effort.