"Just to throw you off the scent for a moment..B737 EFIS ... VSI indications come from the IRS's and are not pressure instuments!"
Sorry to hijack the original question, but the 737 IRS's probably get pressure altitude from the ADC's to _assist_ with the vertical speed calculations (as they do on the 747-400 to give long term correction). Try pulling the ADC circuit breakers and see what happens to your 737 IVSI indications.
Regards.
Q.