I'll try to answer your questions.
#1 Most residential panels the circuit breakers are only good for 80% of their actual rating, so with that being said your 15amp breaker is only good for 12 amps of continuous use.
#2 #14 gauge wire is only good for 15amps and should not be installed on a 20amp breaker.
#3 I know your in Canada but in the US National Electric Code does not allow to run type NMB cable (romex) to be run surface on drywall inside of your garage, it has to be protected.
#4 It is highly unlikely that if you want to replace the existing wire to the garage and use it as a pull string if it was installed correctly in the first place.
If you want to have the proper installation of your new central vac system and not worry about what to use and when have a dedicated circuit installed as per instructions.