it depends on which filter you use in vegas. Colour corrector using the wheel method as above for shadows, midtones and highlights individually. Looking at the colour wheel for each value move the centre dot in the opposite direction to the colour on the wheel you want to reduce. If it's yellow and you are using the other filter colour balance then move the green slider down and some of the red.Yellow is the mixture of red and green. There is don definitive values to use so you would need a lot of trial and error here to get the effect you desire

But if it's just a yellow colour you want to reduce in saturation then unfortunately vegas can't do this for individual colours, which i why i use Adobe After effects for all of my colour correction as vegas is a little too basic. If using after effects then use the colour correction>hue/saturation and you can change each individuals colour with a lot better results