Vinyl will fade in time if exposed to direct sunlight. Some types are worse than others. What they call "marine grade" vinyl, for example, lasts much longer. I do not know the difference, but I know that not all is created equal.
Just take a look at any older vehicle convertible top as an example of the process. Use Duragloss 203 on it about every 6 weeks, and you can extend the lifespan a long time. Also, it will make your banner look minty new :-) Just do not use Armorall on it. It actually causes vinyl to contract leading to eventual loss of malleability and cracks.