Hunt down your system vimrc and remove sourcing thereof if you don't like that behavior (or substitute in its place, see VimTip63.
#Visual studio vim mode windows
Result: more Windows like behavior (ctrl-v is "paste", instead of visual-block selection). On Windows the script seems to be getting sourced for many users. Pressing twice replaces all the text of the original selection. You can replace a set of text in a visual block very easily by selecting a block, press c and then make changes to the first line. (Or in the latter case, try with i_CTRL-R_CTRL-O.)