That's an interesting question.
Often we hear that one of the consequences of creating negative kamma is the grief that comes from reflecting on or remembering the evil deeds that one has done.
Are those thoughts vipaka?
A good example is in the Dhammapada, the wrong doer reflects "I have done wrong", here and in the life to come. I would guess that *isn't* vipaka for the reason I mentionned in previous post, that a thought containing a narrative about one's wrong deed couldn't be a dhamma - or could it?
I also think of strange mental impulses that arise out of nowhere. Now this sort of confession might be going too far, but I think I'm not the only otherwise sane person who has had the flash image of picking up the knife that is lying on the cutting board and driving it into a loved one's body. (OK, no one is going to talk to me anymore....) To pick a less horrible image, you're sitting having a nice talk with a sweet old relative, and you flash on throwing the tea in her face! Those kind of weird flash images feel like vipaka, they are so sudden and so removed from the way one usually thinks. But they are probably more likely just more proliferation on the visual information, as in the Honeyball Sutta, happening in a flash but still proliferation on visual object, that is processed into "knife" that is processed through proliferation into....and so on.
To look at the honeyball sutta, (MN 18) "Depending on the mind and mind-objects, mind-consciousness arises. The meeting of the three is contact. With contact as condition, there is feeling. What one feels, that one perceives. What one perceives, one thinks about. What one thinks about, one mentally proliferates. With what one has mentally proliferated as the source, perceptions and notions (born of ) mental proliferation beset a man with respect to past, future and present mind-objects congnizable by the mind."
I don't know if that helps us get at whether thoughts are vipaka. THe "mind-objects" in the above formula are, I think. But that could be an Abhidhamma view, as JC points out. I think according to Abhidhamma everything that arises to be cognized is vipaka. (Correct me if I'm wrong.) But the "thinks about" above would be the beginnning of proliferation rather than vipaka. So the flash image of hot tea in granny's face would not be vipaka. (Ah, but what if one had actually done it, and that flashed up.) Gaargh.
Usually I don't pursue these deep topics very far but I would like to keep on this one. Knowing vipaka from the productive cittas that result from response to it is very important, I sense.