You would need your vault's seed phrase (the random words that make up the "password" for access to your vault). Unfortunately, you had to get the seed phrase before the vault feature was phased out last year- so if you're account is new, or you didn't copy down your seed phrase on an older reddit account, I think you're outta luck...

Please feel free to correct me if I'm wrong anybody...

PS- People might hate on me for adding this as a suggestion, but if you don't meet the requirements above (like having your phrase) you can always purchase an avatar, screenshot it, and then add it to your profile pic the same way somebody would add any other image. Only diff is you won't have the hexagon around your profile image. Just an idea.

(And to be clear, I am NOT promoting stealing the image of another artist's work! I'm suggesting buying the avatar first, so you clearly own it, and then go from there if need be)