I think it's to do with resonance in the intake, the intake is designed so that whatever speed the aircraft is travelling at it slows the air to .4-.5 Mach to present it at an acceptable speed to the compressor, any faster than this and it would lead to a surge or stall.
At certain speeds/engine RPM like in the Vulcan and other aircraft, as the engine's mass flow rate increases, the air is being forced down a narrower hole until it hit the divergent section at the speeds mentioned above. Imagine blowing over the top of a bottle, it has the same effect (a resonance).
The ViVGs are all about engine start up and up to certain engine RPMs to allow the compressor to operate as close to the blade stall margin as possible without causing them to stall. As far as I'm aware it does not have any bearing on the howl from the intakes.
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