You need to have VirtualBox guest additions installed in the box your are using to get this working.
If you are using default boxes like debian/stretch64, it does not include VirtualBox guest additions, so you need to install it manually.
To do so you need :
* to add guest addition iso cdrom in VirtualBox GUI and mount it
In your console/terminal :
mount /dev/sr0 /opt/tempdir
sudo apt-get install -y linux-headers-amd64 build-essentials
rm -fr /opt/tempdir
You need to shutdown the vm, modify the Vagrantfile to allow synced folder :
config.vm.synced_folder "data", "/var/www/public_html/wpthemes/wp-content/themes/UberBasic/"
You can start your vm with vagrant up.
After that you will have a full synced folder between vagrant host and guest.
Be aware that if you use vagrant destroy, you will have to repeat all from the beginning.
Or you can also use a Vagrant box with VB guest additions embedded which is more simpler.