Is this a single ticket, not a connection from other flight? I mean if it's purely stand alone V HKG-SIN, there should be a fare rule attached to it, which highly likely will be non-endorseable.
But I am thinking also, since I see you are from YYZ, perhaps you buy a ticket YYZ-HKG AC HKG-SIN-UA in V. If this is the case, the fares should be a special fares from AC, and you should check their restrictions. Perhaps it soesn't necessarily to take UA from HKG-SIN in conjuction with your YYZ-HKG. I am just guessing here.