If you’re using Vagrant to manage your Magento 2 installations, you’ll want to be careful with file permissions. By default, it appears that Vagrant’s synced folder setup won’t allow the apache/web user to create folders with permissions elevated higher than the default sync share.
Dropping in some custom mount options should fix this up, and while 0666 and 0777 aren’t ideal, that seems to be world we live in nowadays.