Hey David,
Without seeing a clip, this is suggestions only.
In the past, I’ve had the best experience using RevisionFX. This was on HD footage with flickering black on footage that had been slowed in post. The grader on the feature film, on getting the footage back, stated that he did not think that it could be fixed – it “shot out” for a 35mm cinema distribution release.
https://revisionfx.com/products/deflicker/
You could also look at BorisFX Continuum Flicker Fixer:
https://borisfx.com/documentation/continuum/bcc-flicker-fixer/
And then there is Post Pace who is suggesting this free idea for PPro, which may work for Vegas Pro too:
No Plugin Method, No Cost
There exists more than one way to salvage a flickering up video clip. The first one is the no plugin method which really blew my mind the first time I learned it! So to do this method:
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Duplicate your footage and stack them on top of each other.
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Go to the Effects Control panel.
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Set the opacity of the top footage to 50
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Go back to the Timeline, move the top footage one frame forward, and DONE! Goodbye, flicker!
This method works wonders, as long as your footage does not include motion or any fast-moving object. You can also move this method one step further by stacking three lines of footage, just remember to set the opacity to 67 and 33 on the copied footage.
https://postpace.io/blog/remove-video-light-flicker-premiere-pro/
Hope that this helps?
Atb
Mads