Hi @Mom2Dogs, we also have a vinyl fence, but without the lattice on top.
I've never used a power washer. We always hand wash our fence (probably about 50 - 6 foot sections) and while I'm washing one, I wet the next panel to be washed with the hose and then spray Simple Green on it. By the time I finish the panel I'm working on (washing and drying it) the panel I pre-treated is pretty easy to clean. Don't know if Simple Green can help you, but it's worth the try, if you have not already done so. Good luck.