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
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.