VA joining either ST or *A would work for me. VA not joining neither does not: because of the lack of a European network, the complexity of different earn rules and status benefits on VA's airline partners, and the potential lack of status recognition from partners when things go wrong.
Given that good *A packages are offered by of A3, OZ and TK (all off whom offer attainable *G if you fly 1 or 2 return flights Australia-Europe in Y with the right airline at the right fare), VA joining ST would actually suit me more than VA joining than *A, as it would offer me ST status.
If VS, VA, VX, EY, EI and some of EY's other associate airlines all join ST in time, that would make ST a much better positioned alliance, much more able to compete with the other alliances.
VA's SQ and NZ partnerships could continue even if VA joins an ST - the Cathay/NZ partnership shows that this is possible if it benefits both airlines.