Its gonna require a custom function that checks your vengeance then runs the formula - relatively easy to make but I'm at work. It doesnt look like there is one readily available (at least not easily searchable) online atm.
An easy option in the interim is just grab a weakaura that shows your vengeance. Since FR and T&C scale linearly with vengeance you can ballpark it pretty easily. High vengeance = high absorb haha