So, it's cannon as of episode 100. Octavia and Vinyl live together. So what are the head cannons? What are they to each other? They do seem to be close.

4635644 They're close, that much is clear, but they're too annoyed with eachother/how they act to be in a relationship, and if they were really close friends/roomates they wouldn't split the house like that.
Sisters.
There's the mutual respect in what they do, but the annoyance is still on par with what I showed when I and my sister shared a room.

4635900

I like to call them that too. I'm gonna have a few stories in my series about her about Octy teasing the heck out of her. When she decides to mentor two kids and help take care of them so their family doesn't get split up, Octavia is supportive, but none the less loves to call her "mommy Scratch" because Vinyl continues to adamantly say she's not adopting (she really isn't). Vinyl gets her back, but she's more into pranks and rough housing (yes, even as an adult) than teasing.

I have no idea where this "Sisters" headcanon came from, and no it's not specefically from Episode 100 because I've seen fanfics with the idea long before that episode was even hinted at happening. I personally prefer them as Lovers, though Friends works fine too.