BDZoom has a nice interview with Yoann (in French). He talks a bit about his working process and his philosophy with regards to the series. Well worth checking out (e.g. via Google Translate)! But if you don’t have the patience for that, I’d like to highlight some interesting figures: he mentions that under Tome & Janry, each adventure sold about 250 000 copies, while they’re now at around 80 000. (Presumably, this refers to European sales of the French edition.)
He also says that he and Vehlmann have been offered another 5-year contract to continue with Spirou (after their initial 3-year contract), and were just about to accept. I believe I’ve seen elsewhere that it’s now official, so we can look forward to several more Spirou albums by Yoann & Vehlmann over the next few years. Good news, I say!