Vanillite debuted in the anime.
And just because Trip obtained a Vanillite does not mean Ash won't catch one. Ash and Trip both have Tranquill, whereas Trip used to own a Snivy.
There are hardly any ice-type pokemon available in Unova, and Trip needed a pokemon that could evolve to show his progress as a trainer. In this case, an ice-type pokemon that could evolve once or twice are hard to come by this gen. A pokemon like Cryogonal that cannot evolve at all would have been too stagnant for a beginner trainer.