import sys
r = nuke.nodes.Read(file = sys.argv[1])

g = nuke.nodes.Vectorfield()
g.setInput( 0, r )
g['vfield_file'].setValue(sys.argv[2])

w = nuke.nodes.Write(file = sys.argv[3])
w.setInput(0, g)
nuke.execute("Write1", 1,1)