Vagrant CLI now has two new commands, vagrant rsync
and vagrant rsync-auto
which can do the job.
Command: vagrant rsync
This command forces a re-sync of any rsync synced folders.
Note that if you change any settings within the rsync synced folders such as exclude paths, you will need to vagrant reload before this command will pick up those changes.
https://www.vagrantup.com/docs/cli/rsync.html