Via tenting when solder masking an entire layer on Altium?
You should re-think your design.


Solder mask is not reliable enough to trust to prevent shorts when there's otherwise a mechanical contact between two surfaces.
If you add solder mask above these vias, that will stop your heatsink from making good contact with the surrounding copper (that you're clearing of solder mask to allow close contact to).


If you can do it, the best solution is to move all the vias on other nets away from the region where you want your heat sink to contact the ground polygon.

In comments you said,


  I am not concerned about the 2nd point since I have plenty of room in the rest of the board to make contact, 


Your heat sink is presumably a rigid object, and its contact surface is flat. 
If you have 3 areas of solder mask (or one that is big enough) you will prevent the heat sink from making metal to metal contact anywhere. There will be gap between the pcb copper and the heat sink surface equal to the thickness of your solder mask.


  I will be using some sort of thermal compound to bridge any gaps.


Compare the thermal conductivity of your thermal compound to that of copper and do some thermal resistance calculations before you choose to use a layer of thermal compound to do more than fill in the air gaps caused by the inescapable roughness of your two nominally contacting surfaces.

Depending on your thermal requirements, you may get away with it, but it's going to be noticeably worse than metal-to-metal contact.