Unfortunately, it appears that vassals are now taken into account when joining the HRE, and so this is no longer possible in Res Publica.

It adds another possibillity to play EU4 that most people would simply not even think of, it is pretty obvious if you would ever look at the button and see it says you only need those relations up but not much peopl who do that before making troops and stuff making you too powerfull for the emperor to allow you in.
It's gonna be a though decision though to either form the HRE and keep Lithuania under Union or form the Commonwealth and use the HRE to vassalize them all and integrating them manually, Paradox should implement a Holy Imperial Commonwealth xD

GOOD to see that it is usefull for some people :)

The commonwealth will now be offically UNSTOPABLE!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!

*Launches game and starts on his road to rule the HRE and the Commonwealth without painful conquest of the empire*

super

Actually, I think, that it is a good guide.

Video guide, lame.