I have a situation where its a green field and the color green exceeds 75% of the vectorscope.
Lets say I want to keep it "that green"- what is the best approach this without the need of desaturate the green?
[stig olsen]"what is the best approach this without the need of desaturate the green?"
Beyond the simple mechanics of isolating it, you are asking about "legalizing" it, as I read the question.
If the tonal impression is what you want to preserve, then you are allowed a certain overage in saturation --> But do recall that 75% is really just a target value for setting up color bars, as the YCyGMRB waveform is specifically set up to that standard so that we can reproduce them. Its like "0" VU tone.
Composite baseband usually allows up to 102IRE (luminance), and up to about 120IRE Y+C. If you do not have a good composite baseband simulation, this might be hard to measure. Its another argument for outboard scopes.
Generally, a tonal value is easiest to re-simulate (and tuck under the legalizer) by adjusting its luminance values -- In yellow and green, dropping the Y component in effect sometimes serves to "intensify" the saturation... so tweaking the saturation down a little more should serve the purpose.