There seems to be a thg.exe
tool that allows to launch TortoiseHg tools from the command-line. With that, you can define a custom Vim command, e.g.
:command! HgLog call system('thg.exe log ' . shellescape(expand('%:p')))
Note: Untested; I don't have TortoiseHg installed; the tool will tell you which arguments it needs.
Alternative
If you'd rather stay in Vim for viewing the log (though the representation surely isn't as nice as in a dedicated GUI tool like TortoiseHg), I can recommend the excellent vcscommand.vim - CVS/SVN/SVK/git/hg/bzr integration plugin.