How should I prepare old subfloor for vinyl plank flooring?
How perfect do you want it to be / How much money and time are you willing to spend?  

The vinyl allure product is at the cheaper end of vinyl products so my recommendation would be to forget about leveling and just make sure that the floor is not out 1/2" or more over 4'.  So take a 4' straight edge or level and move it along the floor and make sure that any gap under the level is less than 1/2".

The vinyl flooring will conform to the rolling nature of the floor to a better degree than most other flooring types and if you don't mind it being a little bouncy in some places I suspect it isn't worth the effort to do anything more than ensure there are no squeaks, there are no large holes and the floor is within the 1/2" difference over 4'.  The floor really just needs to be flat not level and pretty much the only case where I'd go to effort/money to flood a floor with self level is if that room receives no furniture and no area rugs.  If you already know the areas where furniture will sit you can spend more time on the areas where someone is likely to walk and notice the bounce / dipping of the floor.