Can't help on mechanical VSIs - all our aircraft use DADCs.
The requirement per ED-55 is to record data from engine start (applies to fixed wing & rotorcraft). A lot of Boeings (e.g. 767) and other aircraft like the CRJ use the Beacon or Strobe Light Switch(es) as the SOP is lights on prior to start. The back-up is WoffW to ensure FDR is recording in air even if lights are forgotten or switch failed.