Is the VSI acurate at Altitude ? Yes, the vsi Basically measures a leak rate difference between two chambers of a bellow, thus not sensitive to altitude.
As Qavion correctly states, the B737 Classic and NG Derives Vertical speed, not from a static source, but from the IRS system, the "pneumatic" altitude rate IS used, but only to Dampen the indication, if you have ever flow on a B737 when an ADC fails, you will see that the onside VSI needle is very erratic.
Hope this can help you
Brgds
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