Hi D.D... you are quite correct the VSI is a very sensitive instrument. The most likely reason is that it uses 0.45 litres of air in the capacity flask, as opposed to the maybe 10 cc of air in the aneroid capsules of an Altimeter; a factor of x 45 more possible sensitivity.
Yesterday when it was very blustery outside, my vario was moving about even in the house, in tune with the gusts of wind.
I suppose I must add my 2p worth to the Power + Attitude = Performance mantra... This is pretty meaningless unless you can supply figures with the equation, and I don't think you can do that, as the units are not comparable. Also as AF447 found out, it does not work for all attitudes.. i.e. tail first.
Just to prove it is so much drivel, lets put some numbers into that formula...Cessna 172.... 140 bhp + 4 deg nose up attitude = 144 performance units
then 140 bhp + 2 deg n/u = 142 units.... ??????? qed.
.