Any real siding supply can order them for you.
Check out Fypon web site, and here's another company.
There called headers.
If you key word search "vinyl window header" lots of sites come up.
Most come with a bracket that gets mounted to the wall then the face plate gets snaped in place.
On a rough surface like that it's hard to keep the two pieces together once the start to expand and contract. We've use stainless steel trim head screws on the top and bottom to keep them together.