dgVoodoo 2 (wraps old DirectX games to modern DX11/12) or DXVK (translates DirectX 9 to Vulkan). These do not "add" Vertex Shader 3.0, but they translate the calls so modern hardware can run old games better.
In the mid-2000s, projects like or 3D Analyze emerged. These are software "wrappers" that trick a game into thinking you have Vertex Shader 3.0 by using your CPU to do the math. This is the only situation where a "download" applies, but it comes with horrific performance (1-5 FPS). vertex shader 3.0 download
To understand why you can't "download" a shader, you need to understand what it is. dgVoodoo 2 (wraps old DirectX games to modern