Vagrant Synced Folder Defaults: Transitioning Between Providers in Version 2

Direct answer

Assume Vagrant 2.x and a single Vagrant.configure('2') block. To force one synced-folder implementation across providers, declare the default guest path with an explicit type: instead of relying on provider defaults:

Vagrant.configure('2') do |config|
  config.vm.synced_folder '.', '/vagrant',
    type: 'rsync',
    rsync__auto: true,
    rsync__exclude: ['.git/', 'node_modules/']
end

Use type: 'nfs' or type: 'smb' when the host and provider support them. Keep provider-specific blocks only when a provider truly needs a different type; a provider block can override the top-level entry.

Using the same guest path as the built-in default, /vagrant, is what makes this a replacement rather than an extra mount. If your version reports a duplicate mount, disable the default first with disabled: true, then declare the explicit entry.

Likely explanation vs confirmed behavior

The likely breakage when moving off VirtualBox is not that Vagrant forgot the folder; it is that the default implementation changed. VirtualBox normally uses native shared folders, while non-VirtualBox providers commonly fall back to rsync or a provider-native mechanism. That changes both performance and the direction of sync.

The portable control point is the explicit type option. Treat provider defaults as convenience, not as a contract.

Type	Direction	Practical limitation
virtualbox	Live two-way	VirtualBox provider and guest additions only
rsync	Host-to-guest by default	Guest writes do not appear on the host automatically
nfs	Two-way over an NFS mount	Not a normal Windows-host option; host NFS support and permissions required
smb	Two-way over an SMB mount	Mainly Windows-host oriented; credentials and share permissions matter

Keeping files in sync

• VirtualBox shared folders are live in both directions, so host edits and guest edits are visible immediately, subject to guest additions.
• rsync is one-way host-to-guest by default. rsync__auto: true starts host watching on up; run vagrant rsync-auto for the watcher and vagrant rsync --all to force a host-to-guest pass.
• Do not assume guest-to-host rsync is automatic. If guest builds must be visible on the host, use a bidirectional mount type or copy/rsync from the guest explicitly. Verify any upload-style option against your installed Vagrant version before relying on it.

Host/provider conditionals

If the same Vagrantfile must work on Windows and Unix hosts, branch on the host platform and keep credentials in environment variables:

Vagrant.configure('2') do |config|
  if Vagrant::Util::Platform.windows?
    config.vm.synced_folder '.', '/vagrant',
      type: 'smb',
      smb_username: ENV['VAGRANT_SMB_USER'],
      smb_password: ENV['VAGRANT_SMB_PASS']
  else
    config.vm.synced_folder '.', '/vagrant', type: 'nfs'
  end
end

If identical behavior matters more than raw performance, prefer the rsync block above. Do not use type: 'virtualbox' on VMware, Hyper-V, or libvirt; use the provider-native type if available, otherwise use rsync, SMB, or NFS.

Verification

1. Run vagrant validate after editing the Vagrantfile.
2. Bring the machine up with the intended provider, for example vagrant up --provider=vmware_desktop or vagrant up --provider=virtualbox.
3. For rsync, run vagrant rsync --all or vagrant rsync-auto, then create a host file and confirm it appears in the guest.
4. Create a guest file and check the host. It should appear for VirtualBox, NFS, or SMB; it will not appear for rsync unless you use a separate guest-to-host step.

Missing detail

Which non-VirtualBox provider and host OS combination must the same Vagrantfile support? That determines whether SMB, NFS, or rsync is the safe default.