Without your own wheels, visiting the Semois Valley is likely to be frustrating, as even the few daily buses that do operate aren't tourist oriented. To reach Vresse, Alle or Rochehaut from Bouillon you'll need to connect via Menuchenet.
Tiny La Haizette Petrol is the valley's only petrol station between Bouillon and Bohan. It's 2km east of Vresse at the N945/943 junction. It's self service so you'll need a credit card.