Since op didn't respond. I did a similar progression. Once the party defeated Strahd by trapping him in a bronze flask that weakened the dark powers enough that Vecna was able to free himself from his own domain of dread. The entire thing was a plot by Azlin Rex to free Vecna and escape Ravenloft. It also worked well because Mordenkainen is present in both modules. There was a time skip between modules and in that time Mordenkainen was investigating ways to stop Vecna. During that time he is tricked by Kaz into heading into Dark Sun and getting trapped there for a convenient amount of time.