I use the Clear app for Vine ideas only. It lives on my phone, is simple as hell, and whenever I think of something to Vine, I just launch it and type it in. It's also great to archive what you've done to possibly reference or expand on for future Vines. Some of the best ideas pop up after a couple drinks or in dreams, so it's great to have a simple note-taking app by in your pocket or by your pillow.
Keep an ideas list.