Hello again :) As of app-editors/vim-core-7.2.264, it still does not support browsing of these tarfiles. This is a real easy fix and I am trying to send them upstream; will update this bug once I hear back. Thanks in advance for including this in Gentoo. I've tested this with lzma (system) and xz (in a chroot). All that I had to do was find every mention of "bz2" and put the appropriate lzma/xz bits there too. You can test this by trying to open a .tar.lzma or .tar.xz file in /usr/portage/distfiles/ (eix-0.17.0.tar.lzma or eix-0.17.1.tar.xz)
Created attachment 205423 [details] vim-core_tar.lzma-tar.xz.patch Allows vim to open and properly browse .tar.lzma and .tar.xz archives.
Patch sent to plugin maintainer. Received response that had the "lzma portion" only because he could not get xz-utils installed at the time. The lzma portion of his was identical to the approach I took in this patch. No response after I pointed him here. Might be stalled at the moment, but I tried. :)
Okay, after doing a bit of a reorg in vim.eclass, it became easier to just revbump vim-core because of filetype patches and not the other [g]vim packages. I've added this patch to app-editors/vim-core-7.2.264-r1