This article in the Islington Times provides a good idea of what to expect on Vis, either side of the month of August – see here.
But there is little mention of activities. Apart from swimming, (wreck) diving, para-gliding, etc there is an increasingly extensive variety of off-road tours (historical, military, vino-gastronomic, archeological), and a network of trails which are well signed which are tailor-made for mountain biking & hiking with GPS guidance provided – see here.
The season starts in April, with the arrival of the yachties, and gets slowly busier until the beginning of July. Thereafter there is rising crescendo of activity the top of which is maintained throughout August; following which the locals slowly subside, exhausted but richer, until Regatta weekend mid October, when calm is restored once more.