  We derive a formula for the first variation of horizontal perimeter measure
for $C^2$ hypersurfaces of completely general sub-Riemannian manifolds,
allowing for the existence of characteristic points. For $C^2$ hypersurfaces in
vertically rigid sub-Riemannian manifolds we also produce a second variation
formula for variations supported away from the characteristic locus. This
variation formula is used to show the bubble sets in \hn{2} are stable under
volume preserving variations.
