How can I access a Vagrant Config object as a string variable in a Vagrantfile?

Question

A Vagrant base box sets config.ssh.username to a certain value. I have a Vagrantfile where I want to use that value in a string. If I create a string like "prefix_#{config.ssh.username}_suffix" the expansion looks like prefix_\#\<Object:0x000000010127e780\>_suffix. How can I get the string value that was stored in the config.ssh namespace?