Everytime you start up the game there is a new chance for an VIP to show up.
In case there is no VIP, just close the game and start it up again.
Rinse and repeat.
Difficulty doesn't matter.
thanks to Ravenous IE:
Just to add, scroll through all the levels, underneath it will say on one of the levels that there is a VIP in that level. If nothing is there do as OP said. :)
https://www.google.de/imgres?imgurl=https%3A%2F%2Fi.stack.im...