Due to a recent and awesome pull request[0] by Jeremy Cline, it is now possible to run Bodhi's unit tests with about 1/10th the RAM they used to take (they went from about 4 GB to about 450 MB!) Because of this, you can now lower the amount of memory required in your Vagrantfile if you wish. There's a PR open to make the default 2 GB[1], but I think you could squeak by with even 1 GB.
[0] https://github.com/fedora-infra/bodhi/issues/1668
[1] https://github.com/fedora-infra/bodhi/pull/1671