Combo cards are always extremely hyped at the early stages of any new expansion. The same was the case with Vannifar, which started its journey at $22 a piece and has now dipped significantly.
Modern players want this card in their Kiki-Jiki and Saheeli lists, but those decks work well without it, so there is no guarantee on Vannifar's price moving forward. That is, it could be a big hit, but it will most likely turn out to be just another generic combo piece.
As it currently stands, the price of Vannifar will drop even further, well below the $10 mark, even if it's good. And if it turns out to be redundant, then expect an even harder push towards the $5 threshold.