I use ctags with my Vim and I rely heavily on C-] and C-T to tag back and froth between various functions in my project.
But if the function is in a different file,
So, this is a feature I would like to see as well.
I'm not sure if it's possible.
What I usually do is
:tabnew :tag somepartsoffunction*
and press tab to use globbing to easily find tag.
Sure, it's not an ideal solution, but it works most of the time.
Unfortunately, with VIM tabbing isn't totally integrated since it's a new feature since 7.0. Most people I know that use VIM weren't even aware of tabs until I told them, so with that, I have a feeling there is no way to do it right now.
(I would love to be proven wrong)