moving a models vertices along its parents axis

[Tristan Mcvean]

Hi all, im not sure what the syntax is to move vertices of a model along its
parent axis.
i can move the vertices about the models own axis.
e.g obj.model("myModel").addModifier(#meshDeform)
total = obj.model("myModel").meshDeform.mesh[1].vertexList.count
repeat with x = 1 to total
theV = obj.model("myModel").meshDeform.mesh[1].vertexList[x]

if theV.x < 0.0000 then
theV.x = 0.0000
end if

obj.model("myModel").meshDeform.mesh[1].vertexList[x] = theV
end repeat

that moves any vertices with a value of less than 0.0000 on the x axis to
0.0000 of its own orientation.
Is it possible to move any vertices that have a value of less than 0.0000 on x
axis of its parents axis not its own.
Please help. Im desperate