Could vanda “oath, pledge, solemn promise” come from *WAD being a variant of WED "bind"? That could mean that *vasta- "pledge, promise" could be a variant of vesta- which was repurposed as "wed, marry" and derived from BES later in the game.

Submitted by Lokyt Thu, 11/12/2020 - 19:32

In reply to by Lokyt

Hmm, NDEB - NDAB look more like a conceptual change, but RATH - RET can't be (and the resemblance is undeniable) and BAL - BEL are definitely sound.
So as much as I'd still ascribe these pairs more to the fickleness of Tolkien's linguistic taste than to him consciously establishing in-world relations, from the in-world POV you may be right.