I won't recommend purchasing Vitality potions if you are a free-to-play player. If you start replenishing your vitality with dark crystals, you won't be able to exchange them to Red Nucleus for rolling on promoted character banners. Sure there are Gold and Black nuclei, but they will feature the standard characters that can be obtained from many sources in the game. Most of the drops in the game are RNG-dependent, and some of them feature standard characters. I would highly advise you to save up your dark crystals to wish on new characters so that you can get a taste of new abilities. Even if you explore all the areas the game has to offer, you have to consistently complete daily missions to obtain Nuclei as a reward. Ultimately, it depends on how you approach investing your resources in your character, but I would recommend you save up your resources for the late game or new characters that will launch in the future.