See the new page about how to do just that on ePubTechReviews here where you'll see I've got two videos already uploaded.
Yes, there's a lot more to come, video-wise, to get you from Vella to Vellum.
Here's the Vella story I'm converting from here (for reference).
And this is the homepage of Vellum. It's a great desktop app. I did choose to get the eBook AND paperback combined version.
I've been using Vellum for years - and I love it!