Translation:The seminars are based on his vision.
It just doesn't sound right in English. You can say I base my ideas on that. Or the seminars base their existence on research. But each time you have to follow the word base with an object. If there's no object after the word base, you use the passive voice "are based" or "is based". Such as, my understanding is based on how much I have used Duolingo.
I agree about this particular sentence in isolation, but it might be possible to avoid the passive voice if we knew who created the seminars based on his vision. For example, "The committee based the seminars on his vision" or "He based the seminars on his [own] vision." To avoid the passive voice there needs to be someone to perform the action.