You can make use of the foldcolumn. The fold column is an optionally visible column that can be put between line numbers and text. It is generally used to show markers to indicate level of folding, if any. If you want to use it to create separation between line numbers and text, but don't want to see the folding markers, just set the foreground color of the fold column to be same as the background color. So something like this:
:set foldcolumn=3 (or 4 or 5 or what4ever. . . )
:hi FoldColumn guifg=black guibg=black
That should do it.