I have a Vapor Trail as well and I use a 32oz. Gatorade bottle. It fits nicely into the side pocket and lies somewhere between a Platy and a Nalgene on the bombproofness scale. And the price was right. I prefer a rigid bottle over the Platy because it allows me to refill quickly and easily as I cross streams. I own a couple of Platys but I only use them in situations where their loss would not ruin my trip. I've had two failures. One was at a bottom seam and the other was when I set it down on a cactus needle in Anza-Borrego State Park. Granted the latter was my fault but it would not have happened with a Nalgene or a Gatorade bottle.

I don't know the exact difference in weight between a 1 liter Platy and a 32oz Gatorade bottle but I'll gladly pay the weight penalty for some peace of mind.

In the near future I plan to experiment with a homebrew hose setup for the Gatorade bottle. I can reach the bottle with the Vapor Trail still on, but getting it back in is a tad dificult.