Define multiple disks inside Vagrant using VirtualBox provider. It is not immediately obvious how to achieve it, but everything you need is there.
Display custom message after the vagrant machine is started.
Use vagrant provisioner to update SSH authorized_keys every time the virtual machine is started.
Configure Vagrant to ignore SSL certificate behind corporate proxy.
Configure the Vagrant virtual machine to use a specific proxy using the vagrant-proxyconf plugin.
Define the environment variable to use specific Vagrantfile.
I have already presented an easy way to build Vagrant Debian base box using qemu, but on some platforms it is better to use VitrualBox as it is not possible to use kvm accelerator.
It is time to look into build process of Debian Vagrant box, create one using packer utility for creating machine and container images and alter the whole process it a little bit.