vertex position node

How would you use vertex position for alpha in this case?

If you want the vertex position, you could simply use the World Position node, which, when used in the vertex shader, is the vertex position, otherwise it's the fragment position.

If you want to have transparency based on vertex painted colors, use the vertex color node and connect it to the Alpha.

Hm,haven't known that  it can work this way,wil try.

Sorry for incompetence in this question.

It works for my situation.Can close this thread.

thanks.

No problem! Glad it worked out :)