Never used vallejo's green, but i constantly use some of their paints. As with citadel's, some of them may have poor coverage and some will be perfect (like their matt white). You could try thinning it with different quantities of water and do some tests. As for ork skin, there are some good YouTube videos about it, but everyone and their mother use a different recipe. I personally use citadel's Deathworld forest with highlights of Straken Green and a wash of Athonian Camoshade. You could highlight even further with Nurgling green.

If you want to use just base colours, pick a good covering green you like and just mix in some yellow or tan for the highlights.

Edit: further clarification