A lot of vamps (a repeated section of a song usually occurring at the end) use a I-IV chord progression. Take for example, the song "God is a Good God" in the key of Eb. When you get to the call and response section "Yes He Is-Yes He Is", it bounces between the Eb chord (I) and the Ab chord,( IV). You can play almost anything that sounds good in your right hand as long as you hit those two notes in the bass on the down beat. I'm going to go out on a limb and say a vamp is usually only two chords, anything more would probably be called a progression.