Vinyl flooring over concrete.

I wouldn’t recommend fixing 5.5mm plywood directly onto concrete. Plywood is intended for timber subfloors, and if it’s bonded or screwed to concrete without the correct method, it can lead to movement or moisture issues. The best solution is to bring the concrete up to the same height as the plywood by applying a suitable smoothing or self-levelling compound. This will create one continuous, level surface ready for the sheet vinyl. Before doing this, make sure the concrete is clean, sound and dry. If there’s any risk of moisture, a damp-proof membrane (DPM) should be applied first. Once the concrete is level with the plywood, the entire floor will be suitable for the vinyl, giving you a smooth, long-lasting finish. If you’re unsure about the condition of the concrete, feel free to send some photos and I can advise on the best preparation.

A latex screed over the concrete is the way to go. Fixing plywood would be the wrong way to go

Do not fix plywood directly onto the concrete. Bring the concrete section up by approximately 5.5–6mm using a suitable flooring-grade smoothing compound. The correct preparation would be: Check the concrete is sound, clean and sufficiently dry. Moisture-test it; install a surface DPM first if moisture readings require one. Mechanically clean the concrete and apply the specified primer. Apply a smoothing compound suitable for use at around 6mm thickness, finishing it perfectly flush with the plywood. Treat the plywood/concrete junction carefully so it cannot crack or show through the vinyl. If the junction is a genuine movement joint, it shouldn’t be covered continuously—the joint must be carried through the floor finish or positioned beneath a seam/profile. Also use flooring-grade plywood and the correct fixing pattern on the timber sections. For an 8ft × 8ft concrete area, you’ll likely need roughly three 20kg bags of smoothing compound at 5.5mm, depending on the product and how level the concrete already is.