In the 80s and 90s, the "center" of SV was Mountain View and surrounding cities, with the VCs mostly in Menlo/Palo Also.
In the last 10 years, as companies have decided to increasingly be based out of SF, the radius of SV is expanding, and a lot of that expansion is to the north + east: Richmond, Concord, Walnut Creek, San Rafael, Mill Valley - making SF the center of the action.