"Vitamin D" lays down a smoky little 4 bar groove at 96 BPM. The chord progression is derived from the key of G and resolves to the tonal center of D, the V chord. The chords are D13 - C/D - Emi7 - D/E - Ami7, making this a V - IV/V - VI - V/VI - II progression.
I like to use my picking fingers for grooves like this so I can pull out the upper and lower voices of the chords using my nails. Improvisation would sound great using D Mixolydian, E minor pentatonic and B minor pentatonic—there lots of choices, but these are a good starting point.