Sorry, but I've never heard of the vacuum index of the membrane used for desalination. In the articles I read, I never encountered this term. The following terms are most often encountered: Vacuum membrane distillation for desalination, vacuum pressure, membrane permeability, feed temperature, and liquid side heat transfer coefficient (fluid flow).
If you have an article that talks about the vacuum index of the membrane, please share it.
Bachir Achour thanks for your remark, I mentioned the vacuum that is usually obtained when the fiber of the membrane is completely broken under the effect of e.g. traction and fluid pressure, also this vacuum is observed in the failure of composite materials.