You can use it full strength or diluted. I would start diluted first. Really depends on the mission and how bad your vinyl is. To really take advantage, get a nice soft bristle scrub brush to get its full cleaning power. The brush is just as big of a key as the cleaner.
As mentioned just make sure to use a good conditioner/protectant after the fact.
CURRENT: 2008 SANTE 210
2000 Mastercraft Xstar
2004,2005 Ski Centurion Avalanche C4
2003, Ski Centurion Cyclone
99,2000,2001, 2002 Ski Centurion Elite V/Hurricane/lightning
88 2001 Nautique