Awhile back I bought the VGC foundation paint set (titled extra opaque) and have used them, and I still use them today. The majority of these paints do match up, but some of them just plain do NOT. Here's the chart for comparison based on the box you get when you buy the VGC Extra Opaque Set:
VGC EXTRA OPAQUE------CITADEL FOUNDATION
Heavy Skintone---------Tallarn Flesh
Heavy Red -------------Mechrite Red
Heavy Violet------------Hormagaunt Purple
Heavy Blue-------------Mordian Blue
Heavy Bluegrey---------Astronomican Grey
Heavy Grey-------------Adeptus Battlegrey (Not a match at all)
Heavy Green------------Knarloc Green
Heavy Blackgreen-------Orkhide Shade
Heavy Warmgrey--------Dheneb Stone (Not an exact match)
Heavy Kakhi------------Gretchin Green (not sure on this one.)
Heavy Ochre------------Tausept Ochre
Heavy Goldenbrown------Iyandem Darksun
Heavy Orange-----------Macharius Solar Orange (a notable difference)
Heavy Brown------------Khemri Brown
Heavy Siena------------Calthan Brown
Heavy Charcoal---------Charadon Granite
As you can see, there are a few colors missing. VGC didn't translate all the colors from the Citadel versions. The colors missing, that you'll need to stick with Citadel with, are:
Necron Abyss
Fenris Grey
From what I've seen, these aren't matched as well as the regular VGC line to the regular citadel paints. And the one's I pointed out (especially Heavy Grey) don't match that well at all. Although, I've not noticed too much of a difference in strength, comparing VMC and Citadel Foundation Paints. They already have plenty of pigment in them.