I have never bothered with VB cuttings so can't help, they are so free with their self seeding I just pot up the little plants and overwinter under cover. I also used to take the really bottom shoots which nearly always have a few roots on them and treat them the same. Another trick for a really good showing of blooms was to give them a good clipping early on and the plants bulked up. Always worth bringing a few in under cover as a bad winter will wipe them out as they are really only half hardy perennials.