There is no contract for Coven, it's a NIS game, it's from their own company. So clearly it's a different situation. Kadokawa can stipulate that they want both versions brought over, and NISA has to oblige if they want to keep a good relationship. Since Coven is their own title, they can do whatever they want with it (like port it to Switch and not localize the Vita version).
Coven not coming over on Vita has everything to do with the Vita market drying up and NISA's disappointment with sales on the platform, and nothing to do with development issues. Insiders like AniHawk have basically confirmed this.