How to get feed output of a grunt task to another grunt task?
You can use grunt.config.set to set the value directly (or to another property and using it with grunt.template if you need to use the value multiple times.)
exec: { create_mock: { cmd: 'cd ~/MyProjects/project/vip-quickstart && vagrant ssh -c \'sh /srv/www/wp-content/themes/vip/the-theme/bin/mock-post.sh\'', callback: function(err, stdout, stderr) { grunt.config.set("penthouse.singular.url", stdout); } }},