As Deadly Vanity resolves, you choose which creature or planeswalker you are saving. Once the choice is made, there is no window for players to respond before the rest of the creatures and planeswalkers are destroyed.

You must choose a creature or planeswalker if there is one on the battlefield.

Because Deadly Vanity does not target the permanent you choose to save, you may choose creatures with hexproof or protection from black.

You may choose any creature or planeswalker, even one controlled by an opponent.

You may activate Selfless Glyphweaver’s ability only if Selfless Glyphweaver is on the battlefield. Only creatures you control as that ability resolves will gain indestructible. Creatures that come under your control later in the turn will not.