The `Mesh` documentation tells you how you can create a new mesh. If you are changing the number of vertices, or how triangles are formed from the verts then you'll need to create a new one.
If you need a new vertex C midway between A and B, then just add them together and divide by two. If C is 40% between them, make a direction vector between A and B, and add 40% of this to A. I guess your problem is the maths to do the vertex creation or edits.
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