sexual images ---> sexual thoughts ----> dopamine release
Imagination of a porn addict just removes the first step. I've gotten the 'high and hung over' feeling from imagination alone,personally so YES imagination can be just as bad as porn especially if you're just starting. Your imaginations will be influenced in large part by the porn you watched. So if that's the case there's not much of a difference - some say the only difference b/t your imagination and just watching porn is the mouse.
Of course, everyone's imagination is different, in terms of how much you can focus on creating the scene and imagery in your mind and how vivid everything feels and looks in your head - but just know that even the fantasies and memories, etc. are just ways for your brain to get its dopamine hits, this will stop, however, if you let it just wither away and allow your brain to rewire.
As far as 'vanilla sex' scenes- Im sure those dont do as much damage, ill admit, but the thing is, as an addict, you know your history. It all started out as vainlla sex scenes, and at some point that wasn't good enough. Who's to say that wouldnt happen again - where that vanilla sex bores you- and how do you deal with the grey area of what point does vanilla turn into pornographic/fetish?
The main thing of the reboot is this - AVOID ARTIFICIAL SEXUAL STIMULATION. Imagination absolutely constitutes this - even vanilla artificial sexual stimulation is still sexual stimulation and IMHO is best avoided. Sometimes it helps to realize what real horniness feels like. I'll admit that I'm not terribly experienced with women, but I have been involved with a couple recently and I can tell you it feels noticably different from the sort of horniness you feel from porn. It's almost moreso you're thinking about what you can give as opposed to what you can get.
Hope this helped