If you are laying flexible tiles on adhesive, an underlay is not required. However, for medium elasticity and rigid materials with interlocking joints, it is recommended to use an underlay in most cases. It is important to familiarise yourself with the manufacturers’ recommendations, which can be found in the tile instructions, on the packaging or on the manufacturer’s official website.
These sources usually state that the use of backer board is recommended. This means that the tiles can be laid without backing, but backing increases the life expectancy of the quartz vinyl, especially if the substrate is uneven, because this softens the load at the joints.