You should also keep in mind that some women will experience vaginal orgasms far easier than others. There have been surveys that indicated that a majority of women never reported experiencing an orgasm purely from vaginal penetration.
Personally, I think the cause of this is partially genetic, partially due to a lack of skill on the behalf of many, selfish males, and partially (to a large extent) due to the intimate emotional/psychological frame of mind that many women need to ‘let go’ to properly experience such an orgasm. I also believe that this third determinant, that of women being unable to let go, is exacerbated by the second determinant, that of men being poor, unaware, lovers.