So here's something I rarely see gone into with Vinyl, probably because her adult persona is interesting for factors other than cuteness. What do we think her childhood was like?

Most of the time, if it's brought up, it seems assumed she just had a generic childhood, with two loving parents who encouraged her passion for DJ style music. However, ever since the 100th episode, a new theory has began to circulate about Vinyl and Octavia. In the episode, the two acted familiar, but not like lovers per say. More like VERY close friends, or rather, family. Vinyl could just move her head a few times and Octavia knew exactly what she wanted. If they're older this could imply full fledged long term marriage, but sisterhood seems pretty likely too, especially if they're not that old.
But here's the thing, Octavia and Vinyl have virtually NOTHING in common beyond gender and an interest in music. Coat color, mane color and style, stature, music style and even pony race. MAYBE the two came from a mixed racial union, but even then you would expect them to share SOME traits.

My best guess, if they are family, one is adopted. I have no reason to believe it's not Octavia who was adopted, but I always have it be Vinyl instead. In my stories, she and Octavia are actually pretty close, Octavia being one of the ponies who helped her, once helping her get adjusted to a new family, and helping essentially rescue her from orphandom/foster care, and further sticking by her when she lost her voice. Not every close relationship has to turn sexual, and I always paint this two as probably CLOSER than most couples.

You can read my rendition of her childhood in "Our Stories" here:
https://www.fimfiction.net/story/384608/2/our-stories/vinyl-scratch-i-love-my-family More on that might be coming, we'll see.

What are everyone else's theories on how Vinyl grew up?

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Possible! I mean, who's stop you. I've written Toola Roola into a lot of my stories as a friend of Rarity and Trender's, but now she's a filly in Ponyville and I still don't care! Except that canon filly Toola is an adorable scamp and I need to write stories about her too!

Wait, I'm getting off topic. Vinyl's childhood. Right.

The background I use in my stories is that Vinyl is a spoiled upper class Canterlotian permanently slumming it, and Tavi had a rough childhood on the mean streets of Trottingham, and is looking to move up to the finer things in life. This is for irony, and also is excuse to give Tavi a cockney accent. "Vinyl, love, could you be a dove and fetch me my cello?" kind of thing.

Eventually, the show will go on so long they'll do detailed backstory for everybody, and it will be awesome, but also suck because all of our headcanons will be jossed.