The bash completion installed by app-editors/vim doesn't work for files/directories with spaces. See Bug 417717 for more details.
:!mkdir /tmp/test\ test :! ls /tmp/te TAB completes in VIM - Vi IMproved 7.4 (2013 Aug 10, compiled Nov 1 2015 01:07:39) Included patches: 1-769 Modified by Gentoo-7.4.769 In which version can you reproduce the problem? Can you provide a minimal example? Please provide more information why this should be a gentoo bug instead of a bug upstream.