Starting the CDK on OSX always fails. Whether it was already started from CLI or not.
The vagrant up command is properly executed, you can see the output in the console, but it ends up with a message "Server CDK Server Adapter at localhost failed to start."

I traced the problem back to:

VagrantPoller.parseOutput(String[]) line: 220	
VagrantPoller.onePing(IServer, String) line: 160	
VagrantPoller.onePing(IServer) line: 138	
VagrantPoller.getCurrentStateSynchronous(IServer) line: 114	
CDKLaunchController$2.run() line: 222	

In parseOutput, the value of lines is :
[1457536005,,error-exit,Vagrant::Errors::ProviderNotUsable,The provider 'virtualbox' that was requested to back the machine\n'cdk' is reporting that it isn't usable on this system. The\nreason is shown below:\n\nVagrant could not detect VirtualBox! Make sure VirtualBox is properly installed.\nVagrant uses the `VBoxManage` binary that ships with VirtualBox%!(VAGRANT_COMMA) and requires\nthis to be available on the PATH. If VirtualBox is installed%!(VAGRANT_COMMA) please find the\n`VBoxManage` binary and add it to the PATH environmental variable.]

this is not properly parsed as it later goes into this block

		Collection<VagrantStatus> stats = status.values();
		if( stats.size() == 0 ) {
			return IStatus.ERROR;
		}

And the message in the lines is lost completely