This may be a stupid question. But what causes the vast glub again and in the newer updates, (Spoiler I Guess?) How did Sollux die from it/how did they save him from it??

Sollux died because he had not yet entered the Medium before it was released and also because he was a yellow blood and only fuchsia bloods can survive it. His Derse dream self woke up and took his place after Feferi kissed his dead body. I explained the Vast Glub already here.