This article will explain you how to add new disk in virtual machine have provisioned with Vagrant. Follow the steps below:

1 – Export default Environment Variable
$ export VAGRANT_EXPERIMENTAL="disks"
2 – Edit Vagrantfile
Add this syntax to your vagrant configuration file:
config.vm.define "dblab#{i}" do |node| ... node.vm.disk :disk, size: "50GB", name: "extra_storage" end
3 – Reload
$ vagrant reload