I have completed the game and most levels finished on "insanity" level. There is a guy in the discusion section who has achieved even more than me and came up with the same result. We dont know what has an effect or not but i believe you have a higher chance if you completed the game?

Anyhow: Start the game. If you can rescue a VIP there is a small sign above the level "VIP" where the VIP is to find. Regardless of which difficultiy you have mastered this very level you can still go down on easy mode and still find the VIP and its probably the safest option.

If you dont see any VIP. Close the game and start again. VIPs will show up. It varies between 5-20 starts (estimated). You need to completely end the game and return to your desktop. I completed this last achievement within a day like this.