Stick with the manufacturer's suggestions on the packaging of the vinly comp. Tile. If you're unsure, check out their website.
There are good concrete leveling products out there, but in addition to that, you may need to use some sort of primer.
We can help further if you'll tell us:
1. What is the condition and level-ness of the concrete?
2. What brand of vinyl comp. Tile are you using? (Exact name and SKU would be helpful.