You might try some of that 3M spray adhesive, put a bit on the back of the decal and some on the siding let it get a bit tacky then press it down & hold it until it sticks good.
Be sure to clean the siding with some alcohol first.
After it sets a bit you may need to rub some of the excess dried adhesive off from around the edge of the decal.
If you try it, let us know if and how it works.
Best of luck, Jim
Jim & SherrySeward
2000 Residency 3790 v10 w/tags 5 Star tune & Banks system Suzuki XL7 toad