This must be done when you are first kicked out of Castle Volkihar, if you continue the quests for Dawnguard after leaving Castle Volkihar, the vampires will be hostile the next time you re-enter the castle.
Go to 3:14 to see how to re-enter Castle Volkihar.
This is for those who want to get Heal Undead Spell Tome and want to continue with Dawnguard questline, this was done on the PS4 for TES V: Skyrim Remastered Edition.
What's required
Wooden Plate
Whirlwind Sprint or just run
Some gold (Obviously)
Level 40+ Restoration Level
This was done without Mods