Beaten dawnguard and i wanted to get back into the soul cairn so i went to fort valkirah or whatever and could not get back into soul cairn. i also tried as a vampire as well but i got the same results.

Also i followed the dawnguards questline and still have serana with me.

try leaving the area with the portal and coming back as a vampire

Yeah that was the problem , i guess when you enter the cell as a human the game will mark you as that as long as you're in that cell even if you're turn into a vampire in there, but if you re enter the cell as a vampire lord then you will have access into the soul cairn. thanks mods can close this.

Can you only go in the Soul Cairn as a Vampire? I'm only a few hours in.

if you chose become a vamp then you cant go back in you had to have chosen lose your soul to go in as much as u want