Is there a filter that will allow me to shoot outside in the sun on auto? I've tried some Neewer Nd filters and while they help stop the whole image from flickering outside, sometimes the sky and light reflections are still blown out. There's also some vignetting sometimes.

Short answer: No.

The filer(s) used are entirely dependent on needs. Do you need to Control highlights? Maintain a constant frame rate? etc.  As the amount of light changes, a filter change might be required to maintain whatever your controlling for.