Erm your description of MSAA isn't quite correct, it has nothing to do with vertices and doesn't increase vertex processing cost..

It's more similar to supersampling, but without the higher pixel shader cost (the pixel shader still only runs once per "display pixel", not once per "sample" like in supersampling).

A pixel shader's output is written to multiple (typically 2, 4 or 8) samples, with a coverage mask deciding which samples are written (this coverage mask is all 1s inside a triangle and a combo of 1s and 0s along triangle edges). After rendering to the MSAA render target is complete, an MSAA resolve operation is performed which merges samples into pixels (and this gives you the smoothed triangle edges).