You cannot directly "import" a VirtualBox VM into Vagrant.
Basically you'll have to package the existing VirtualBox VM to a box
and then add it to vagrant.
NOTE: a box is tarred VirtualBox Exports (could be gzipped).
You would be doing
Package the existing VirtualBox VM (VDI format)
vagrant package --base mybox --output /path/to/mybox.box
See more here =>
http://docs.vagrantup.com/v2/cli/package.html
Add the box to vagrant
vagrant box add mybox /path/to/mybox.box