You can test it a month early and get 13 months on the new certificate, if that helps.
The vacuum test is not totally infallible, but it usually finds anything significant. For some of the really tricky slow leaks, you sometimes have to resort to dye in the refrigerant, or the sniffer gadget.
One tip, make sure you use the aircon regularly, even in winter - it can be useful for demisting. Running the aircon, at least for a few minutes every week, distributes the PAG oil around the system seals and gaskets, helping prevent leaks.