Variance At Completion (VAC) is the difference between what the project was originally expected (baselined) to cost, versus what it is now expected to cost. Every month, our vendor is required to report this total on the project as a whole and on key deliverables. I'm used to seeing the numbers reported and how to calculate them. I'm not asking for the Cost Performance Index (CPI). I want to know how far over or under we're going to be compared to the budget.
The formula I memorized for the PMP exam and the same formula I use to calculate VAC today is: Variance At Completion = Budget At Completion - Estimate At Completion (VAC = BAC - EAC)
So, I ask myself,  why is there no VAC definition and  VAC formula in the PMBoK?