Starting with vim-8.0.1298 (and possibly earlier) the paren match highlighting within quoted strings stopped working. It appears to have been corrected as of vim-8.1.0115. See this resolved bug I logged upstream: https://github.com/vim/vim/issues/3190 I'm not sure offhand what would constitute a more recent stable release of vim 8.1 to add to portage, but at the moment I'm running gvim/vim/vim-core version 8.1.0181 with no issues out of my local overlay. This was just a matter of renaming the existing 8.1.0034 ebuilds.
A few notes about the changes made to the bug: To be clear, there never was any app-editors/vim ebuild for any 8.1 except app-editors/vim-8.1.0034. Also, as I originally entered it, the issue only exists in versions < 8.1.0115, not <= 8.1.0115.
The bug has been closed via the following commit(s): https://gitweb.gentoo.org/repo/gentoo.git/commit/?id=c2229a0f270617e0b1c467df538e8a7cda51f31f commit c2229a0f270617e0b1c467df538e8a7cda51f31f Author: Tim Harder <radhermit@gentoo.org> AuthorDate: 2018-09-20 04:54:24 +0000 Commit: Tim Harder <radhermit@gentoo.org> CommitDate: 2018-09-20 05:30:52 +0000 app-editors/vim: version bump to 8.1.0412 Fixes: https://bugs.gentoo.org/661070 Fixes: https://bugs.gentoo.org/661254 app-editors/vim/Manifest | 1 + app-editors/vim/vim-8.1.0412.ebuild | 313 ++++++++++++++++++++++++++++++++++++ app-editors/vim/vim-9999.ebuild | 6 +- 3 files changed, 318 insertions(+), 2 deletions(-)