To copy all the content of a file in Vim to the clipboard, you can use the following command:
:%y+
Press ESC key to quit from insert mode to normal mode, then press : to type in the command above(%y+).
Basically, this command yanks (copies) all the lines in the file to the system clipboard. The % symbol indicates that you want to operate on all lines, and y letter is the yank command. The +  sign tells Vim to use the system clipboard.
See also:
