I think it can be "fixed" by adding more warm water to dilute the ACV which sounds like you used way too much.
I don't use any acid in my henna. I just mix henna with warm water until I have a nice thick warm mud and let sit at least 6 hours and color release is find. The so called acid is only if you want the color to release immediately, like within 1 hour or so, but if you have time to let it sit then no acid is needed. Some even say you don't need for instant release, that release occurs anyway with warm water. Period. Check out "The Henna Thread" at
www.longhaircommunity.com which is predominantly caucasian and they discuss same henna issues as on this board and some ladies there mix their henna with warm water, wait 45 minutes and apply and they get gorgeous color. It was that board that led to simplify my mix.
I also don't put any oils or terps or condish or honey or whatever in my henna mixes. Just warm water and it comes out Bling Bling every time. I DO use a deep moisturizing condish afterwards and that's where I put all the extra oils and things, but my henna mix is simply pure henna and water.
I've been a henna girl now for over a year and I've used all kinds of henna, my first was that waay too expensive henna from Catherine's site which was great and informative and all, but her henna is just too expensive, knowing what I know now. I wised up and started ordering from
www.fromnaturewithlove.com where the stuff is like 1lb (450grams) for $6.95 and it is just as good as any. I've also used brands from local indian stores (hesh, reshma, karishma, etc.) 100grams for $1.99 and its all been good. . . .