Not sure about drop rates, but vlad can be your main until you find higher elemental attack increases.
An unconditional 30% elemental attack to absolutely everyone is probably the best event eidolon you could have since the others give more, but to one element.
You can actually use her until you get gacha ones (mainly 100% eidolons) since she covers all elements... but if one boasting 40% or 45% shows, I'd still replace her with that one later on.
In leveling and drop grinding teams though, leave her in the main slot and try to find a 2nd, unless your team isn't strong enough (unlikely unless its AQ4)