Underlayment for vinyl plank flooring
That I am aware of T&amp;G is not available in 1/2". 5/8" is thinest, and if carpet is the choice of finish, it should be 3/4". But it doesn't mean it will be.

You will need to set the screws or any nails that are above the floor, and check if the subfloor is flat enough for the manufacturers requirements for their floor. Usually it is something like no more than 1/8" deflection in a 4' span with a straightedge. If that is all in order, I would add just a layer of 1/4" underlayment, not plywood, although underlayment I typically use is plywood, difference is it is rated as an underlayment. Underlayment does not have the voids in the laminations of the plywood so if a table leg or heel presses over a void, it will not poke a hole in your finish floor.

If the floor you have now is squeaky, now is the time to resecure it with 2" screws. Typical nailing/fastening for underlayment is 4" on all edges, and 6" in both directions everywhere else. I like using a pneumatic narrow gauge stapler for this, it makes fastening go real fast.