that's a good video Nick and it may help Squampton!
However it sounds like at least one of two things - possibly both are causing this vertical bow.
The versatips are tapered so there is less weight at the front. The leader and fly has very little weight and can't sink much on it's own. The back of the tip is restrained by the floating line so the result is the pronounced U you observed. You can see this even with a full sinking line fishing still water. Many sinking lines are density compensated so the tip of the line uses a coating with more mass than the belly and the line sinks uniformly.
Also there are often upwelling currents in deep turbulent water - a boiling effect as water moves into deeper water hits a rocky bottom and is deflected up to the surface. This can lift an unweighted fly and leader right to the surface in several feet of water while the heavy sinking portion remains 4 or 5 feet below.
What to do? Keep the leader sort but you have done that. Weighting the fly with lead wire, a good size bead or barbells may help. Use a heavier tip. Get some T-8 or T-11 and make a tip from that. Some folks cut the the taper section of the tip off - turn your 15 foot tip into 10 to 12 feet. A radical approach is the put a folded nail knot loop into the front of the tip (
http://www.youtube.com/watch?v=5HlF7pR8CHE)
and loop the front of the tip to the main part of your versaline so the heavy back section becomes the front of the line. I have done this without problem but some folks have trouble with a hinging effect.
Hope these suggestions help.