Do I need an underlay for gluedown vinyl?

No, you don’t need an underlay with gluedown vinyl flooring.
Gluedown vinyl is installed directly onto the subfloor, so a separate underlay is not required. However, the subfloor must be perfectly level, hard, and stable. Any unevenness should be smoothed out beforehand to ensure a beautiful and long-lasting result.

An underlay is only needed for floating click vinyl floors.
It provides the right support, cushioning, and sound reduction, and helps to bridge minor unevenness in the subfloor.