Nuns (even in the broad sense; properly speaking, there's a difference between nuns -- largely contemplative orders, I think -- and sisters, those in the more familiar teaching an nursing orders) don't get the habit without taking vows. Postulants don't take vows. After the six-month or so postulancy, they take simple vows (i.e., for the length of the novitiate) and actually get a habit -- "take the veil," as it were, though novices generally wear white veils instead of the black. After the novitiate, they take solemn vows -- for life. But simple vows are real vows.
Thank you for your thoughtful reply.