The consonant sounds in "fed" and "vittles" are both made by putting your top teeth on your lower lip and blowing air through your teeth.

If we make these sounds the same way, then what is the difference between them? The difference is not what we do with our mouths, but what we do with our voice.

When we make a /f/ sound, we do not use our voice; the sound is quiet. Put your hand on your throat and make the sound /f/. You should not feel any movement in your throat.
However, when we make a /v/ sound, we do use our voice. Put your hand on your throat and make the /v/ sound. Can you feel the vibrations?

Watch videos with this sound! Here you can see a close-up of how your lips should look when you say /f/ and /v/. (These videos will launch a separate video viewer to play them. Make sure you have a plug-in for video clips.)