So, since there's a lot of misunderstandings and missing complete guides, here's my best effort to provide one:
First thing first, however, you need a few things:
-You need to be Rank 3 in the guild;
-You need to have Titan Step unlocked;
-You need items - 1) 50+ Healing Herbs; 2) Anti-Poison Status Accessories, as many as your team's members;
Now, onto the guide:
1) Accept the Immortal Quest in the Guild;
2) Report to the Nun at the Trade Post south of the Capital (the first settlement outside the Capital) during daytime;
3) You need to go to the Forest with the Abandoned Church during nighttime - you need to move east to where you previously fought the Werewolf and you will find a circle spot for the quest, interact with it and watch the cutscene;
4) Next you go to Port Town and speak with the guy with the boat. The last option takes you to a shortstack expert that tells you where to go next - beyond the Titan Step there's a tent-like spot that is the 'Adventurers Camp'. You speak with the character in front of the biggest tent that tells you to speak with a character in the southermost section of the camp, the one west to the well. She will tell you to go in the forest, then take to the Temple north of the forest and retrieve an item from a boss;
5) I suggest doing a first save here as the forest and the Snek Temple are tough if you don't use the Cheat Sword. Once you defeat the boss, you can use the magical circle to get back to the entrance and then return to the camp. Speak with the one that told you about the temple and she will give you the means to brave 'further' in the forest (which is the forest tile west to the first one) - you may have to unlock the path leading to west of the first forest tile to make it to the deeper area;
6) In the new forest, you need to look for a way to go northward to the Poison Swamp - fight your way through the mobs and then you will make it with ease;
7) Before entering the swamp, save. You need to equip the Anti-Poison Status accessories to prevent a perma-poison status before entering the Swamp, and you still need to be careful while crossing the purple river since it will deal instant damage. Take a few steps, heal, more steps, heal again (repeat this every six steps) - you can reach the exit going north, then east, and then north again;
8) Save before entering the castle (possible bug tied to quest door). Once entered, hear some noises and you need to use the staircase on the left to get to the upper floor, go around and go right. The door has a quest circle. Enter it, watch the Scene I aptly call 'Gooning Vampire Princess Introduction';
9) More cutscenes (Priestess 'learns' about 'Netorare' and 'Cuckquean' by girlboss girlfailure Vampire Princess), MC and Priestess stay over and thus the scene 'Vampire Princess' totally non-rapey first time' occurs;
9B) Important to save after the Cutscenes since the final step of the quest will bug the Bar with looping quest's final scene!
10) Once done with the Vampire Princess manor, she will tag along to see what this ghoul situation is about - go back to the Forest of the Abandoned Church and return to the spot where it all began - go through the last bit there and then return to the capital;
11) Enjoy the last scene with VP being an utter social moron with a deep horni mindset (hardwired to just you rn), and then you get her as a perma-follower to party-up with!
Current known scenes for VP:
1) Gooning Vampire Princess Introduction;
2) Vampire Princess' totally non-rapey first time;
3) All Girl-Follower Defeat Scenes with all available mobs for VP;
If there's any question about the guide, do not be afraid to ask!
