So, there was this story I read ages ago. I can’t remember the name exactly, but I remember the rough story.

It was of Twilight, but something happened to her and she became Princess Rest after being reincarnated? There are vampires involved and somewhat of a relationship between Luna and Rest, though I don’t think it’s been openly realised yet?

I don't think this is the one that you are talking about, but your description sounds kind of like a story called "Crimson Night". Twilight is seriously injured in battle, but Luna saves her from certain death . . . by turning her into a vampony. It does turn into a TwiLuna story. Even if it isn't what you are looking for, I suggest you give it a read.

https://www.fimfiction.net/story/24426/crimson-night

I, too, remembered that one, but couldn't find it in my library. Thankfully, a bit of Google-Fu came to help and this should be it:
https://www.fimfiction.net/story/381252/once-more-with-feeling

If anyone's interesting in the process: I googled "Princess Rest" site:fimfiction.net. Exclude the period ending the sentence. Adding the quotes around Princess Rest makes sure Google looks for exactly that phrase and not any page containing both words, "Princess" and "Rest", and the site: part specifies to only look for results on the given site. Can be helpful in cases such as these.

7518463
Thank you so much!

7518207
I think that one’s in my Read Later folder, I’ll probably get up to it sometime this month, thank you!