I don't get where you get that a referendum would be demanded from the Basque Country independist party (bildu), that would totally go against what they have said about doing all that is possible to avoid having Vox in the government.

Also, that would be a stupid move, right now the basque population clearly doesn't want independence, they want their special rights respected and their language to remain protected, which is what both the right nationalists and the left nationalists say, one focused in social issues and the other one on the industry.

Demanding a referendum for the basque country would be political suicide for any party, I'd bet that not even half of the Bildu voters want independence, they just vote the local left party because they feel like they will defend their interests, besides the independence thing, the best.

Edit:

Also, I wouldn't bet on Vox going down on reelections, the reason PP went up is because it got a lot of Vox's votes, people that hoped for a right government with PP. They have failed, so there will be several voters that will return to their original parties. The same thing will not happen to the left because there's more urge now to avoid the right, I hope.