Vamping backup works great behind vocals, but is especially useful when backing up a mandolin player's solo. When they stop "chopping" the banjo player needs to fill that role. It does require a bit more from the fretting hand than rolling backup - 4 Finger chords are the norm. While playing with steady time is the ultimate goal, you may need to spend a while just moving from one chord to another and getting used to how it feels.
Pay close attention to the banjo tab! There are fingering suggestions for the fret hand and pick hand. Banjo chord fingering suggestions are listed just above the tab using numbers "1, 2, 3, and 4". These represent index, middle, ring and pinky fingers. Fingering suggestions for the picking hand are listed just below the tab using "T, I, and M" which represent thumb, index and middle.
Check out the licks to learn some "chord inversions" and more advanced Scruggs style vamping. Chord inversions invert chord notes to give a different sound for the same chord notes. It's a great way to make things interesting when vamping on one chord for a while, and they can also provide tasteful transitions in between chord changes.