Is it possible to override vertex shader of another shader?

For example, i would like to use Unity Standard Shader and add my own vertex deformations.

Is it possible to override a vertex shader or extend or blend two different shaders?

No, but you can download the source of the Standard shader from the Unity download site and add in your own vertex modification to it.

See the “Normal Extrusion with Vertex Modifier” example shader on Unity - Manual: Surface Shader examples .