title: Vim: Custom grep finds files but does not open them
body: <p>I have my grep program defined as follows in my vimrc:</p>
  
  <pre><code>set grepprg=grep\ -R\ --exclude=*~\ --exclude=*.swp\ --exclude-dir=vendor\ --exclude-dir=node_modules\ --exclude=tags\ --exclude=tags.vendor
  </code></pre>
  
  <p>When I do a search, it brings up a list of results just fine, but when I do <code>:copen</code>, although it will list the files, I am unable to open them.</p>