What kind of op amp is it? how do I calculate the VA voltage?
simulate this circuit &ndash; Schematic created using CircuitLab

Vin+ is set to a fixed voltage Vout changes to null the input difference "error".

Iin to both Op Amp inputs is close to zero so we neglect this current.  Same for Q1 when OFF.  Thus R1 has (almost pA~uA?) no current flowing thru it when Q1 is OFF.