How common is it for a vsd surgery to fail and for there still to be leaking?

Common. If a tiny leak (stitch channel) is observed. Rare if a relevant big leak is found.
Failure uncommon. Failure of a vsd repair is very uncommon. Occasionally however there will be a small residual leak, which is insignificant, and may close completely later on.
It is uncommon. Most vsd repairs result in complete closure. Defects lower in the more muscular part of the wall are often hard to define and are at higher risk to have incomplete closure. Rarely a suture may come loose resulting in a residual leak. Usually the residual defects are small and do not require additional surgery.