Are those stretches of the Olvidado worth the hassle?
I had to look at a map to piece this together, but now I get it. So, you would go León to La Robla - Boñar - Vegacervera - Buiza and then continue back on the Salvador. (Robla to Boñar is actually “backwards” on the Olvidado, but you need to take that backtrack to get to the starting point of the Vegacervera alternative, at least that is how I see it).
Boñar-Vegacervera -Buiza are some of the days of real mountain beauty on the Olvidado. La Robla-Boñar not so much. I think that if you want a taste of what is in store for you if you walk the Olvidado, this is a good way to dip your toes in the water so to speak.
La Robla to Buiza is a lot of asphalt (but it does have the selfie moment with the wolf up on the mountain) and is not the prettiest part of the Salvador, IMHO, so I think it’s a good trade. And the closure of the café/bar at the Buen Succeso Ermita means that you can’t get an astonishingly good pincho of tortilla there anymore, so it’s one less thing you will miss!
In terms of stocking up, Boñar has plenty of shops. There is a Día in Ciñera, which is not far from Buiza on the Olvidado. So I think you will be fine in terms of buying food.
Sounds like an interesting idea, and I am sure you will not regret the day in or out of Vegacervera, they are both wonderful.