Screen printers need vectors that are already separated into spot colors, with fonts converted to outlines, at the final print size. A PDF from your designer is a start; a print-ready file is more work than that.
Every color should be a named Pantone spot color, not a CMYK mix. Screen printing uses one screen per color; if colors aren't separated, the printer has to redo the separation.
Fonts must be converted to outlines. Otherwise the printer either has to own your font or substitute one.
Overprints and knockouts must be handled explicitly. Two colors overlapping without an overprint setting will mix on press.
Bitmap effects (drop shadows, glows) become impossible in spot color. Either flatten them or design without them.
A professional vector conversion returns a file that a print shop can drop onto press-ready films without another round of prep. That's the standard we hold ourselves to.