Condition for setting up a vagrant synced folder

Question

I want to setup a synced folder in vagrant, but only if the mountpoint already exists in the guest machine.

This is my synced folder:

  config.vm.synced_folder "./typo3conf", "/var/www/typo3conf",
    owner: "vagrant",
    group: "www-data",
    mount_options: ["dmode=775,fmode=664"]

Now before this synced_folder is setup, I would like to check, if the directory /var/www/typo3conf already exists and ONLY THEN sync this folder to the host.

Background: Typo3 will create the folder during inital setup, but will not execute the setup, if the folder already exists. As soon as it is created by the setup process, it should also be synced to the host.