Checked, yes, the current release
vendor directory does exist with bin, composer, johnpbloch, oscarotero, roots and vlucas dependencies installed under it.
I’d assumed this was a problem with upgrading from Sage beta 3 to beta 4 on my production server.
However I’ve now also tried it on a fresh DO staging server and the issue remains the same.
The setup is the latest and greatest:
Bedrock WordPress 4.9