A rust stain is an iron based stain, a leaf stain is also an iron based stain.
What works best for removing rust stains is oxalic acid. However because it is an acid it won't be very kind to the threads used to stitch details on the awning
, the vinyl surface is not so much of an issue for harm as those lines of stitching. Threads are also the first things to deteriorate from UV damage, the weakest link in the product.
Oxalic acid is in typically in products made to clean rust stains off of vinyl siding and plastic shower stalls. It is sometimes found in a not overly concentrated solution used for marine vinyl cleaning on boats. That would be the best source to look for a cleaner that removes rust stains from vinyl as there are often outdoor marine vinyl surfaces on boats and rust is also an issue in those situations. Those types of surfaces also have stitching on them. So focus your search with products made for marine use for the best chance of finding one that will do the least harm.