I have a split level home and the family room is @ 4 feet below grade on a slab. We are changing from carpet to vinyl planks.
We know a vapor barrier is needed but one person is also suggesting a heating grid under the vinyl.
Our frost line is only @ 41 inches but recent winters are not as cold as several years ago and I do not expect freezing below the slab. With carpet the floor never felt cold.
We do not want to add cost and do not want to increase electric bills so would prefer not to do it but if it is really needed now is the time.
FYI: in SE Michigan, it can get cold outside but usually cold spells are short and overall winters are moderate.