Hello
reply to 1. Open Virtualbox Settings, check the virtual machine directory, you can change it.
then destroy and recreate will put the VM on the new path.
reply to 2.
dd with zeroes may not use much space, several layers are there that will try to be smart. Filesystem, the Virtualbox disk, etc.
if you want to use space, you need to create files that are not zeroes, ie put something big like an iso on on same directory of Vagrantfile, then on the VM copy that multiples times.
Om the host the disk.vmdk will start growing if the VM start using it.
Vagrant is a tool that automate lots of things, but is good to know whats happening under the hood, so please feel free to send all the questions you have, and we will be more than happy to help.
Alvaro