This may sound strange, but sometimes your best bet is to put the spotlight above the water (with no volumetrics), rather than underneath it. I used this technique in Bryce to get an underwater lighting effect in this pic that I could find no other solution for. It would probably work the same in Vue. The only catch in your situation is to make sure the spotlight doesn't hit the actual edges of the pool.

Another suggestion might be to use point lights instead of spotlights where half the point light is against the wall under the water plane. This way, it should only light the inside of the pool and not the outside. I've never had much luck in getting spotlights to work well in these situations.

ShadowWind