You want a gate for voltage ...but at zero current? Is there any current through the Vout connection? What connects to Vout, just a DMM voltmeter?
A simple way to perform your task is to connect a resistor "R" between Vin and Vout. This makes Vin=Vout, but only as long as Vout isn't loaded down with an external resistor to gnd. Next, connect your mosfet from Vout to gnd (Drain connects to Vout, Source connects to gnd.) When turned on, the mosfet will "short out" or "pull down" the Vout terminal, giving Vout=zero volts. Now go find a "logic level" mosfet which will turn on when Vgs = 1.0V. Connect Vflag to the mosfet gate.
On the other hand, if you're connecting Vout to an LED or motor or something, then Vout must supply significant current, and your "R" resistance would have to be fairly low, and when the mosfet turns on, the resistor gets hot.
Other solutions: use a mechanical SPDT relay, with a sensitive coil which turns on at 1.0V. The COM relay terminal becomes Vout, the N.C. terminal goes to gnd, and the N.O. terminal goes to Vin.
Or, the professional version: use mosfet-based "analog switches" which can connect two signals together, or short a signal to ground. For example, here's a Digikey search.