How add opacity to Particles/VertexLit Blended Shader?

Hi guys,

I am trying to make some clouds with particles. I found the Vertelix Blended to be a good one. The problem is that I want to make the clouds cast its shadows when the sun is behind them. I tried all kind of particle shaders but the only shader that seems to work is a trully opaque one. However, the clouds dont appears or are terrible viewing. I checked both cast and receive shadows option in the particle system, but they dont work. I dealed with color over time property too, but the clouds still dont block my sun light (directional light).

I am new to unity and dont know much about shader programming. Can you help me please? Thanks in advance.

Curious about the same thing as well. I can access it under the Standard shader with the _Color property, but there is nothing in the default VertexLit Blended Shader to gain this control