As has been said, but is often not explained often enough.... Power+Attitude=performance, but that is a vertical performance as well as a horizontal one.
WIth respect to the VSI, the majority of the time (and I think this is what CowsGettingBigger is alluding to) it is too sensitive to be part of the primary scan, often leads the student into chasing needles rather than developing an appropriate scan of the control and performance instruments.