clip could be output to mpv player right from script itself using output() and using subprocess modul. VSEdit might need clip.set_output() at the end.
Code:
import vapoursynth as vs
from vapoursynth import core
import subprocess
clip = core.avisource.AVISource(r'C:\video.avi')
#import shutil
#mpv = shutil.which('mpv')
mpv = r'C:\tools\mpv.exe'
mpv_cmd = [mpv, '-']
process = subprocess.Popen(mpv_cmd, stdin=subprocess.PIPE)
file_handle=process.stdin
clip.output(file_handle, y4m = True)
process.communicate()