Pegaso, Andrea, Benieto amongst others all have dioramas and vignettes. I've done a Pegaso Roman v Germanic vignette and an Andrea Waterloo French Cuirassier v Allied Hussar vignette... both were really well done and fun to paint. For Napoleonics I'd go to Andrea and Benieto, for medieval and ancients I'd check Pegaso.
I'm sure there's others but that's a start.
Good Luck
Colin