We're working out what we think - we'll add our view here!
CNOC Outdoors says:
The Vesica is a collapsible, reusable and light water bottle that behaves like a hard bottle for easy drinking on the move. Simply carry 1L of water and stow away when empty.
We've taken our experience from the Vecto and made a robust, light and easy to use bottle so you can have a handy tool to stay hydrated. Featuring a collapsible body with a stiff neck and bottom, the Vesica can stand when full with water or air, held for drinking and collapsed when empty.
Using the same thread as the 42mm Vecto, the Vesica can be used as a dirty water bottle for filters like the BeFree by Katadyn. If you like to filter into a container for easy drinking, use the Vesica with the Vecto to create a fast, light and easy to use filtering system.