Project Vector to Plane v1

project_v1.py

# get plane normal
axis = cmds.createNode('math_AxisFromMatrix', name='proj_AXI')
cmds.connectAttr('plane.matrix', axis + '.input')
cmds.setAttr(axis + '.axis', 1)

# get dot product between vector and normal
dot = cmds.createNode('math_DotProduct', name='proj_DOT')
cmds.connectAttr('src.translate', dot + '.input1')
cmds.connectAttr(axis + '.output', dot + '.input2')

# scale normal
scale = cmds.createNode('math_MultiplyVector', name='proj_MUL')
cmds.connectAttr(axis + '.output', scale + '.input1')
cmds.connectAttr(dot + '.output', scale + '.input2')

# subtract result from vector
proj = cmds.createNode('math_SubtractVector', name='proj_SUB')
cmds.connectAttr('src.translate', proj + '.input1')
cmds.connectAttr(scale + '.output', proj + '.input2')