The only problem is that when you use multiple instances of vim and let your TWM handle the tiling, you lose the sharing of registers and command/search history amongst the open buffers. I believe you can solve this by creating a vim server, but I've never tried it.
I try to keep a balance between the amount of tiling that vim and my TWM do by clumping similar parts of a project into one instance of vim. Even then I'll have the different instances of vim reside in their own WM tags. I use window splits in vim extensively and only use tabs when I'm working on small monitors (netbook) - even then I'll have at least two windows per tab.
With this setup you have only one GVim instance that works as usual. Switching between this single GVim X11 window and other X11 windows from other programs is your WM's business, not Vim's.
Same goes for browser tabs. Just a kludge for the rarity of good window managers.
And most other applications that use tabs inside them.