I'm guessing what the problem is from your description since you don't have pics. Maybe the vinyl is too thick and is trying to return to its flat position? So the adhesive is not strong enough to keep it flat? You could try gentle heat on the vinyl to soften it, like from a hair dryer. Keep the dryer moving, don't stay on one spot too long. Disclaimer: I've seen this done but haven't done this myself.
Or...........
Instead of vinyl try spray painting decal paper and cut out the shape then apply like a regular waterslide. Conforms to sharp bends more easily than vinyl.
Or.........
Use a stronger spray adhesive for the vinyl. I don't have a recommendation for that.
I do have a heat gun, and thought about doing that, but was afraid they would shrink a bit too much.
I also thought about applying waterslide paper over it with nothing printed on it, but wasn't sure that would hold the vinyl down, and maybe it would start peeling up and breaking through the waterslide material. Do you think that would hold them down?
I ended up using a drop of E6000 on one winglet, and thin CA on the other. After applying the E6000 on one side, I used some of those little plastic spring clamps to keep it closed up while it cured - they are both sticking now, although the side I used CA on; some of the label has already started peeling up. The side with E6000 seems to be working better... I've got pics attached, and yes, I do believe they are so thick that they do try to straighten themselves out, and lift up on the thinner (bottom) side.
I didn't think about just printing those black strips on waterslide paper instead of vinyl, but I figured the vinyl would look better. I may just do that and print those two winglet strips on waterslide decal paper and maybe use some Micro Set/Sol.
Just brainstorming here - and thanks for all the replies - I had NO idea you could use water/soap on vinyl sticker decals, so I haven't ever used that method. I'll have to see if there is a video on youtube or apogee's site on how to apply vinyl sticker decals using this method.