This is what I did on a spare dash. I think I could do better next time to make the transition less noticeable.
1986 4Runner Refresh
Dude! That looks sick!! I bought this stuff specifically for filling in cracks in vinyl dashboards. It’s like a super flexible bondo I guess. Was planning on filling over the top of the original vinyl, sanding smooth and painting the entire dash. I get that it would be smooth in the spots I filled but I think it will look a hell of a lot better than the cracks. Now that I see your approach I think I might go that direction. Where did you get your vinyl from??