I believe you want to be tiling on something like cement board or ditra not directly over plywood.
Can hardboard be place on top of vinyl tiles.
How to 4 laying hardboard and plywood 1 use a chalk line to mark two lines at 90 degree angles to each other across the centre of the room f2.
Surely ply has better moisture resistance.
Two layers of 5 8 plywood at least then use cement board.
If you are planning to tile over the hardboard make sure that the rough mesh side is facing up to give more of a key for the tile adhesive.
4 loose lay more boards up down and across the room.
Fill low spots on the floor with patching compound applied with a putty knife.
I was planning on using 6mm plywood for a solid base for vinyl although i see most websites recommend hardboard.
Hardboard is a very poor poor product to use for preping a subfloor.
Stir ready to use floor patching compound with a paint stirring stick.
It is not a good idea to lay ceramic or stone tiles on suspended upper floors unless you are sure that the joists can carry the extra weight consult a structural engineer for this.
It can be confusing because many leveling compounds are marked as useable on a variety of surfaces including linoleum and vinyl.
If your flooring is already thick enough needs to be stable to prevent cracking later.