I noticed your comment yesterday and meant to reply, and then forgot about it.
So, you're right there is no "built in" support for Overrides on a per-project basis in Vagrant. There is some built in support - it will source local Vagrantfile files and wrap them around the project specific one (so e.g. my $VAGRANT_HOME/Vagrantfile is setup to tweak the defaults for the Vagrant-Hostmanager and Vagrant-Parallels plugins, and this then applies to all Vagrant machines that don't otherwise change those settings).
Probably what you really want though is the ability to override per-project, per-dev.
This is where it helps (a lot) if you know ruby, or if you're willing to learn a little (which was my case for this problem).
So, you're right there is no "built in" support for Overrides on a per-project basis in Vagrant. There is some built in support - it will source local Vagrantfile files and wrap them around the project specific one (so e.g. my $VAGRANT_HOME/Vagrantfile is setup to tweak the defaults for the Vagrant-Hostmanager and Vagrant-Parallels plugins, and this then applies to all Vagrant machines that don't otherwise change those settings).
Probably what you really want though is the ability to override per-project, per-dev.
This is where it helps (a lot) if you know ruby, or if you're willing to learn a little (which was my case for this problem).
I've put the basics of this concept into a gist here: https://gist.github.com/stephenreay/2afd4205e76836f20e176722...