does the manufacturer of this light also make one to fit 6" x 6" vinyl posts?

You can use this post cap on 6x6 vinyl posts. This cap comes with adapters to fit many sizes.

Have not seen them, but if you have a flat top railing, you can do a faux post of 4x4 and mount on that. You could also try to cut the thickness of the 6x6 back to fit the light.

Yes...the light comes with a kit to be able to use it on different size posts.