What happens when HCl is added to acetylene in presence of mercuric ions?
I'm not entirely sure but I'll have a go. 

As a strong acid, HCl will be dissociated into Chloride and protons in water. I'd suggest that the mercury will first associate with the acetylene forming an Hg- Pi complex which will weaken the triple bond. Then the chloride ion will attack one side of the acetylene and the electron density one of the triple bonds will form a new single bond, first to the mercury ion and then this bond will hydrolyse and pick up a proton. This is similar to an oxymercuration reaction but with an attack from chloride rather than water. 

The resulting vinyl chloride is more electron poor than the starting species so should should undergo further 'chloromercuration?' slower than the starting species. I would suggest that the selectivity is then derived from the milder conditions this reaction must be run at. Probably you have to force acetylene to react with HCl without a catalyst by heating the hell out of it (if it happens at all) therefore the addition happens twice.