TL;DR version:
Go into LIGHTS -> Sunlight -> Caustic Photons and change the EMIT RADIUS there. The units are in your model units. And the center starts from the sun's location.
Full explanation:
Vray projects a beam with an 'Emit radius' when rendering caustics. Where that edge ends, you'll see a cut-off line.
In 3ds max you can set this emit radius' direction and size visually. Buuuht, not here.
In Sketchup the sun is the emit source, so depending on time of day(!) you'll have to fiddle with the emit radius.
The attached image has three swimming pools: at 34' from the origin of the blue axis (0,0,0), then again at 60' and again at 90'.
At 1:00pm the sun catches all the pools nicely, but the 2d and 3d pools aren't getting enough caustic photons to show caustics properly (top half of image), because the emit radius is set to 700, which doesn't extend far enough.
NOTE: The units of this model is inches, so that's 700 INCHES or 58.3'. The yellow circle has a 59' radius, and you can see the 'dark line' cutting through the pool.
The second render shows the emit radius set to 1080" (90'). This time the third pool is captured and begins to render caustics.
Tadah!
vray 3.6 sketchup causticss affected by emit radius setting under Sun->caustics menu