Using a little bit of film clutter for transitions (overlaying some end burns and light leak style film flashes) can be a nice transitional element, especially if you’re working with the vignette. But if it doesn’t work to suggest an old movie kind of feeling, I think a little bit of diffusion, or as was already mentioned, a glow – works nicely. Don’t know what plugins you have, but Too Much Too Soon is a set of sharware plugins that has 2 great diffusion filters.
https://www.mattias.nu/plugins/
Also, if you want to emulate a lab process, like bleach bypass, you can use the 55mm digital film tools plugins. I think it’s now called DFX 2 and distributed by tiffen. You can download a trail at the website: https://www.tiffen.com/dfx_v2_home.html
That particular set of plugins has some really nice pro mist filters and other tools that can help you soften the look. But you don’t necessarily have to go soft or slow with flashbacks. Look at all the “Saw” movies. Their flashbacks are generally sped up and extremely harsh looking. But then again, that might not be useful if you’re not working on a horror movie promo. 🙂
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