Vritra's reprint is also late enough to be after Tiamat. Tiamat actually has a viable passive (+40% element and... an unknown amount of +double) and... really good stats for an event eidolon. Summon effect's a barrier that eats 750 HP on a 10 turn cooldown.

Like, I bring up Rahab's summon effect because it has possible niche uses, but that's also me digging around for reasons to pick her up at all. It helps that the competition for your AP here is Medusa, who has even fewer reasons to get. But as far as a water eidolon actually important enough to keep an eye out for, Tiamat's the next one on the list.

When Vritra comes up again, she'll be paired up with Sphinx. Sphinx's event is way, way more important to focus on than Vritra's.