Devoiced vowels vs just skipping them?

I keep seeing beginners completely drop the "u" or "i" sounds in certain words, making them sound super choppy. Is there actually a tiny breath of a vowel left (like a whispered state) or is it totally deleted? My textbook says "devoiced" but on TV it sounds like they just skip it entirely depending on the speed. Anyone got a good rule of thumb for this?