Improve support for Vagrant VM definitions

plugins/vagrant/_vagrant

@@ -56,7 +56,7 @@ __box_list ()
 
 __vm_list ()
 {
-    _wanted application expl 'command' compadd $(command grep Vagrantfile -oe '^[^#]*\.vm\.define *:\([a-zA-Z0-9]\+\)' 2>/dev/null | cut -d: -f2)
+    _wanted application expl 'command' compadd $(command grep Vagrantfile -oe '^[^#]*\.vm\.define *[:"]\([a-zA-Z0-9_-]\+\)' 2>/dev/null | awk '{print substr($2, 2)}')
 }
 
 __vagrant-box ()