First, open VirtualBox and select the Virtual Machine you want to add port forwarding;

Then, click Settings and select on the Network tab;

In the Network window expand the Advanced section and click Port Forwarding;

In the Port Forwarding Rules window, click the + button and fill out the new rule as such:

Name - SSH
Protocol - TCP
Host IP - leave blank
Host Port - 2222
Guest IP - leave blank
Guest Port - 22

Once you’ve filled that out, click OK to save this rule.

ssh -p2222 USERNAME@

In Vagrant, it uses port forwarding in VirtualBox to implement its ssh command.