OS X Lion Virtual Machine on Yosemite?
Lion works fine as virtual machine - as well as all Mac OS X' since Snow Leopard* - in all of the mentioned apps. VMware/Parallels usually offer a better system integration and better performance though - with the downside that they aren't available for free.

To get the OS X Lion installer, check your purchases in your App Store account. If you have downloaded it previously on the MacMini, it should be available there.



*: It's a bit tricky to install Yosemite in VirtualBox and performance is sluggish.