You can see what vagrant instances are running on your machine by running
$ vagrant global-status
id name provider state directory
----------------------------------------------------------------------
a20a0aa default virtualbox saved /Users/dude/Downloads/inst-MacOSX
64bc939 default virtualbox saved /Users/dude/svn/dev-vms/ubuntu14
a94fb0a default virtualbox running /Users/dude/svn/dev-vms/centos5
If you don’t see any VMs running, your conflict is not a vagrant box (that vagrant knows about). The next thing to do is to fire up the VirtualBox UI, and check to see if it has any instances running. If you don’t want to run the UI, you can:
ps -ef |grep VBox
If you have VirtualBox instances running, they should be included in that output. You should be able to just kill processes that have VirtualBox in their output. One problem is that one of those processes seems to exist to do keep-alives. Just kill off the highest VirtualBox process. If you have a VirtualBox image running but vagrant doesn’t know about it, some Vagrant directories may have been deleted manually, which means Vagrant loses track of the instance.