If it won't hold its charge for a year, I think there's a problem. Vacuum testing isn't the best way to test, but it's quick and easy. To be honest, I'm a little sceptical of the automatic machines that the likes of KwikFit use. Obviously, they're geared towards (a) selling top-ups and (b) doing the job quickly. I had my Landy topped-up by KwiKFit last year and they said it held a vacuum just fine, but by the start of the warm weather this year, it needed re-gassing.
Holding a vacuum is the direct opposite of the way a/c works, which is under pressure.