You can activate one loyalty ability of each planeswalker you control during each of your turns, whether that ability is one it always has or the one it gains because of Elspeth's Talent.

The set of creatures affected by Elspeth's Talent's last ability is determined as the ability resolves. Creatures you begin to control later in the turn and noncreature permanents that become creatures later in the turn won't get +2/+2 and gain vigilance.

The last ability will resolve before the loyalty ability that caused it to trigger. Notably, if that loyalty ability creates creature tokens, those creatures won't be on the battlefield yet to get +2/+2 and gain vigilance.