I am prepping a floor for tile. I have removed the old vinyle flooring.There is no sub-floor, just concrete slab. Some of the glue from the vinyle is still on the slab and you can see the swirls from the old glue. Must every bit of glue be removed?
I’m assuming you are installing ceramic tile since you also used the term vinyl for what you removed. If the residual glue is securely bonded and it’s thickness is minimal in relation to the troweled thinset you will be applying to secure the tile then no you don’t have to remove every trace of the old glue. Also if the glue is in ridges and won’t come off easily you can also level things out with a coat of mortar or floor patch.