I honestly think there's a much broader issue at play than just VA and *A. The recent announcements by Royal Air Maroc and now Alaska Airlines are the first, new full members of any alliance in almost six years.
There are also the exits by CZ from SkyTeam (due to partnerships with OW airlines plus minority stake investments), as well as the recently announced exit by LA from OW (due to minority stake investment by DL).
If we were to go by past history here, Mexicana had to be alliance-free for a certain period prior to joining OW.
If we go by today's trends of increasing 'cross-alliance' JVs such as NZ/CX, QF/EK as opposed to sticking with Alliance partners, this means that we're not even sure if CZ will eventually be joining OW or even LA joining SkyTeam in the future.
VS deciding to remain alliance free despite being run by DL, just like their baby brother VA is another stand-out.
We're all looking at reasons as to why VA specifically is not joining *A specifically, but it probably makes more sense to look globally at why no airline anywhere and none of the alliances has added a full member in such a long time. Whatever reasons there are for VA not joining an alliance it would seem that VA isn't the only airline and *A isn't the only alliance to be in that position.
IMO, DL will always be the elephant in the room when it comes to VA and *A discussion, considering that partnership goes way back to the pre-Borghetti, Godrey CEO era. Which is before SQ, EY, NZ, et al decided to buy stakes in VA.
Apart from VA mending bridges with NZ, the only other (unlikely) option where VA is considered by *A as a "potential member" in the future, is UA ambushing DL and subsequently buying either the EY and/or HNA stakes in VA. Making it a 'game-changer'. Similar to how DL ambushed AA and purchased a stake in LA.
If DL is the party to buy the HNA and/or EY stake in VA, it would be at that point where "VA joining *A" would be 'finally' considered dead, buried and cremated. SQ at that point would consider looking at selling out.
Saying all that, it's unlikely that any party will be interested in buying into VA for the foreseeable future, considering the recent record low VA share price. HNA had looked at selling out a number of times and wasn't able to secure a serious buyer suggests that VA is not a entity to invest in as of the moment.