Is any underlayment or vapor barrier needed before installing these directly over a concrete basement floor?

An underlayment or vapor barrier is not necessary when installing our 8mm Titan Rubber Tiles over concrete. Moisture does not affect the tiles in any way. If you do have moisture issues with your subfloor, you may notice a white substance under the tiles. This is not mold or mildew but salts from the concrete. These tiles are mold and mildew-resistant, but if any organic material were caught under the tiles or if you currently have mold and mildew issues, you may still have issues with this until rectified.