Can water seeping through the vinyl floor damage my underlay

This depends on your underlay. The Comfort and Transit underlays are made of polyester and won’t be damaged by water.

The Heat underlay is somewhat more susceptible to water due to its mineral base. However, small amounts of water spillage, like those commonly occurring in kitchens or bathrooms, won’t damage your floor or your underlay.

Just make sure to avoid cleaning your vinyl floor with excessive amounts of water and don’t leave liquid on your floor for extended periods of time.