According to the current code, the three possible endings are:
- You have to get Cream's "happiness" all the way up to 50, but not gotten her naked, in order to get the "best ending" with Vanilla taking you to her bedroom where Amy and Sally are already waiting for you so that all three of them can bang the stuffing out of you.
- If you've gotten her naked, then you get the "good ending" where Vanilla takes you to bed and bangs you herself without inviting Amy or Sally over to help.
- If you have not gotten her naked or gotten her happiness to 50, you get the "not-so-good ending" where Vanilla just pays you for watching Cream and sends you on your way.
So if you want to get the orgy ending, you have to finish with a happy, fed, and
not wienered bunny.
And don't ask me why it was implemented that way, because I really don't know. (Though if I had to guess, my first guess would be that it ties into the whole positive/negative morality thing, and was meant to be yet another "secret ending" for people to find if they did the non-obvious, non-intuitive thing and did
not do anything naughty with Cream during the week despite practically every situation in the game encouraging them to do otherwise...?)