Populate directly the Vertex Shader output (Varyings)

Unity Engine
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I can’t find a way to populate directly the Vertex Shader output (Varyings) using a computeshader.

I managed to run a computeshader in the render (unclear but still useful link: https://docs.unity3d.com/6000.0/Documentation/Manual/urp/render-graph-compute-shader-run.html) but I found nothing about “hacking” the output of the vertex shader, populating the Varyings though a GraphicsBuffer.

I could just run a simple vertex shader just reading the data from this buffer then putting it in the output, like o.positionCS = i.positionCS, and so on, but it really feels like a waste of resources/performance.

My triangles are special, sharing specific data among blocks of triangles, thus my need. I could bypass it by using the good old InstanceID, CommandID etc of instancing, each block being an instance

but then, I do continuously read the same data for multiple vertices, AND I do believe that computeshaders, at least in some situations, can be faster than vertex shaders (intrisics, shared memory, groupID and/or numthreads usage, etc).

Thanks for your help!

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EDIT: it appears to be totally impossible to squeeze data between the vertex shader and the rasterizer, as this rasterization step is done right away once the vertices are provided, moreover by dedicated hardware, just using the Z-Depth buffer and thus only storing pixels upfront. It is not a buffer filled with all vertex shader outputs then all processed at once… So my question is groundless.