Don't worry- as evidenced by the Pokemon Dream World, a Pokemon with Vital Spirit or Insomnia can in fact sleep when it wants to. It just won't when it's ready for battle. Vital Spirit sounds like a great ability since you won't get sleepy when you're trying to get things done.
Insomnia, on the other hand, would probably be more troublesome, as it would mess with your sleeping pattern. Be glad you didn't get that.