VAM is maintained by two users, VAM can emualate Vundle (or could If I would spend another 6 hours on vundle (as I did reviewing the issues - finding many duplications). I don't think that stars are the right way to draw conclusions. VAM has been first, VAM has all features vundle has, VAM provides dependency management. VAM has a pool (thus allows to manage the community by softly telling users about alternatives) etc. VAM is nice to windows users because it supports vam.mawercer.de etc.
We know that vundle appears to have more users - but it doesn't mean its the better plugin manager (it caused Shougo to fork in the end).
We are aware of this: https://github.com/MarcWeber/vim-addon-manager/issues/148

The "declarative" design behind VAM is superior: I've created an issue:
https://github.com/gmarik/Vundle.vim/issues/388

One important difference is that vundle depends on vim-scripts, thus cannot install all scripts because vim scripts maps titles to urls without caring about some titltes being used multiple times VAM (vim-pi) use script_id disambiguation.

I don't want to discuss its nice to have a plugin listed in your .vimrc and starting vim doesn't tell you that its missing (vundle case). It matters to me. I prefer declarative approaches in most cases - similar difference can be seen in your UltiSnips version vs my fork. Please keep VAM for those reasons. Supporting vundle also means supporting the dev workflow of vundle (trivial issues not getting processed for 2 years!). This leads to duplication and waste of time.

Summary about issues can be found here at the bottom:
https://github.com/gmarik/Vundle.vim/issues/241

The action we took for VAM was improving the README and preparing that missing features (such as version lock) could be implemented trivially.