I have an old app I need to run. How can i run SL as guest with Yosemite as host with MacBook Air 2011 and vbox? I own SL retail and the MacBook Air is apple, there shouldn't be any legal issues.
You are only allowed to run a Server version of Snow Leopard in a virtual machine.
From Lion you are allowed to run the same guest OS X as the running host OS X - and you are allowed to install a client version of the OS.