fugitive.vim is undoubtedly the best Git wrapper of all time.
This plugin allows you to use it with private stash instances.
Install it as you would install fugitive.vim.
To use private stash repositories add the follow to your .vimrc
let g:fugitive_stash_domains = ['http://mystash', 'http://mystash.mydomain.com']
fugitive command :Gbrowse will now work with stash URLs.
fugitive-stash.vim requires fugitive.vim 2.1 or higher to function.