I think you have a good idea for a tattoo, and if people think you're vain then they are the stupid ones. Judgemental people aren't worth worrying over anyway. Get whatever you want. Whatever is meaningful to you.
I self-harmed for about 9 months, I cut my arms and my legs. But I don't scar easily and I only have one scar on my thigh. So when I'm 18 I want to get a tattoo over it that says "The Shadow Proves The Sunshine". Maybe consider a short quote or just one word, like Hope or Live. Something that maybe has helped you through your experience with self-harm. Good luck.