On his 15-minute break from kaisering.

You are doing God’s work, my son

And sideswiped by the Cat Stevens.

It’s not a question of stopping heat. What he wants to stop is hydrostatic pressure and vapour from his slab mixing with warm air. Without a vapour barrier BELOW his heat source - and preferably some insulation as well OP could be creating ideal conditions for condensation below his flooring in the cold/hot mixing of vapour, especially if a low perm flooring is sandwiching the vapour within and slowing any potential drying upwards. His heating cable will be working full time to dry vapour rather than heat his room space and running full time adding virtually no warmth to the room. OP is better off with a space heater. If this was an above- grade bathroom he’d have no problems. His specific installation over a slab likely leaking vapour is a recipe for ineffective heat that will run full time. $$$

You might get some heat to the touch of the foot for some minor comfort improvement but for any appreciable room heat, which I think OP will need for his application in a garage conversion, the radiant will not provide enough heat for overall room heat it that’s what he’s after. In Ontario building code for new construction for instance, as far as radiant heat as the primary source of building heat, any vapour barrier must be BENEATH the heating source tube or cable to mitigate vapour to be less than 45 ng/Pa•s•m2 . Usually poly is best placed below any R10 insulation foam as well which the radiant heat would be tied on to or a puck panel used. Again, OP isn’t strictly following code I guess (because I bet he’s not even allowed in most municipalities to convert a garage to livable space) but in terms of just getting good bang for his buck, that cable or mat isn’t going to heat much without a thermal mass (being embedded in concrete) or with LVP above it.

Radiant heat should have NO product above it that acts as a vapour barrier as it severely limits heat radiation. LVP will act as a vapour barrier and is not a candidate for radiant heat.