There is no command in "vi" to do this, obviously the select/copy buffer is part of X11, vi has no knowledge of this.
You can open the file in a graphical editor... or use GNU vim, which I believe has the ability to deal with luxuries like this.
@ocicat: There is an internal concept of copy/paste "buffers" in vi, it has no connection with the X11 select/copy buffers.. but vim has a way AFAIK.
Alternatively there is
x11/xclip that allows you to pipe text into one of the buffers, it can also read the files itself if passed as an argument.