Help repairing a vintage dress?

You could try fabric glue. I’ve used it on a couple of things, although never on chiffon, and it’s been pretty successful. I couldn’t say how it’ll work for chiffon though. Good luck!

I’ve seen these patches you can use where you put it in between the layers of fabric and iron over it to meld it together… however i’ve never used this before so don’t know how well it works, maybe you could look into it!