I think you might be getting a bit confused (or maybe im getting confused) Vals will get their carbon from CO2 like all other plants, its only when their is a carbon dioxide deficiency in the water ie. not enough CO2 available, then they derieve CO2 from the hardening constituents of carbonate hardness. This process is known as biogenic decalcification.
Other plants can do this as well, Elodea, Najas, Egeria and Ceratophllum as well as Vallisneria, i cant fully explain what happens but the Ph auctually rises which is counter intuitive, because the plants are stripping the water of the KH, it causes big Ph swings at lights on and lights off and is not at all desireable when this happens, a sign of this happening in your aquarium can be a whitish like power as deposits on the leaves of plants, which is auctually calcuim carbonate precipitating because of the rise in Ph.
Anyway the solution and way to avoid this is to add sufficient CO2 and have a good level of carbonate to act as a buffer in case things go awry.
I hope that sort of explains it, if you look up "biogenic decalcification" on google im sure you will find lots of info.
glad to help
