In Tools - Options - Text Editor select Custom Editor
Here I use gvim, enter in the Editor text box:
C:/Vim/vim80/gvim.exe --remote-tab-wait-silent [file name]
in
C:/Vim/vim80/gvim.exe
Replace with the path of the editor you want to use,
--remote-tab-wait-silent
It is a parameter to use gvim, see the remote parameter for details