To step through this recipe, you will need the following:

Some Vagrant boxes are available for multiple hypervisors, such as the CentOS 7 Bento box we previously used. This way, we can simply choose which one to use.

Let's start with our previous Vagrantfile including customizations for VMware:

Vagrant.configure("2") do |config|
  config.vm.box = "bento/centos-7.2"
  ["vmware_fusion", "vmware_workstation"].each do |vmware|
    config.vm.provider vmware do |v|
      v.vmx["numvcpus"] = "2"
      v.vmx["memsize"] = "1024"
    end
  end
end

How would we add the same configuration on VirtualBox as we have on VMware? Here's how to customize VirtualBox similarly in the Vagrantfile:

  config.vm.provider :virtualbox do |vb|
    vb.memory = "1024"
    vb.cpus = "2"
  end

Add this to your current Vagrantfile, reload and you'll get the requested resources from your hypervisor, be it VMware or VirtualBox.

It's nice, but we're still repeating ourselves with the values, leading to possible errors, omissions, or mistakes in the future. Let's take advantage once again of the Ruby nature of our Vagrantfile and declare some meaningful variables at the top of our file:

vm_memory = 1024
vm_cpus = 2

Now replace the four values by their variable names and you're done: you're centrally managing characteristics of the Vagrant environment you're using and distributing, whatever hypervisor you're using.

The simple fact that the Vagrantfile is a pure Ruby file helps creating powerful and dynamic configuration, by simply setting variables that we use later for all the providers.