Note that in many engines, vacuum is provided by a separate vacuum pump, which is driven by the rotation of the engine or rarely by an electric motor. This is always true for a diesel engine (because there is no intake manifold vacuum) and many petrol engines too (if they are direct injection, they also may not have sufficient intake manifold vacuum).
Even if you have a total engine failure, if you're going down a hill, if you keep it in gear then the motion will be turning the engine still, which will be generating vacuum regardless of whether that is by a separate vacuum pump or the intake manifold (unless the throttle valve also fails or you get a gaping hole in the side of your intake, but that seems unlikely).
So braking should still keep working as normal, as long as you keep it in gear and don't disengage the clutch.
Note that Toyota only introduced vacuum pumps in 2017, well after the changeover to electronic throttles. All plotted unintended acceleration complaints would've been without a separate vacuum pump.
Even if you have a total engine failure, if you're going down a hill, if you keep it in gear then the motion will be turning the engine still, which will be generating vacuum regardless of whether that is by a separate vacuum pump or the intake manifold (unless the throttle valve also fails or you get a gaping hole in the side of your intake, but that seems unlikely).
So braking should still keep working as normal, as long as you keep it in gear and don't disengage the clutch.