I chose to get Vampirism to go in the Soul Cairn. (I guess a mistake) Afterwards, I got it cured, so does that mean I can't go back in the Soul Cairn?

yes, if you chose the vampire method you can only enter the soul cairn as a vampire whereas you could enter it at any time if you chose the soul trap method. Such is the price for being inconsistent.

Yea, I kind of need to work on that.

As Serana's mother states, your body becomes attuned to the Soul Cairn while you're in it, and afterward you can enter it anytime you wish, even when not soul-trapped.