Although vipaka may not always signify specifically karmic yield in the Nikayas, I use the term with that meaning here, in order to avoid awkward constructions such as "karmic yield" or "karmic fruit" and to avoid loading the discussion with the connotations of "result.
Akusala actions, on the other hand, generate apuhha, also known as papa, which, in turn, accumulate until expended in painful vipaka such as rebirth in hell (Evans "Ethical" 522-529).
Moreover, our focus is specifically on the relation between kamma and vipaka as distinct from ordinary, that is, material, psychological, and social, processes.
The Culakammavibhanga Sutta, noting that beings inherit their actions, with their actions as womb, with their actions as relatives (kammadayada kammayom kammabandhu), gives a detailed list of rebirth destinations as vipaka correlated with different kinds of action, or rather habitual behaviors (M III 202-203).
It is worth emphasizing, first, that the vipaka affects specifically the agent who performed the initial action and, second, the analogical correspondence between act and vipaka: killing issues in a short lifespan, harming in illness, unpleasant behavior in unpleasant appearance, and so on.
Punna and its opposite, apunna or papa, have to do with kamma and the vipaka of karmic results, that is, actions and the pleasant or painful results necessitated by the law of kamma, often, though not necessarily, following rebirth.
In other words, in its time the force of punna issues positive vipaka as karmic result, including, though not limited to, a happy rebirth for one who possesses it.
Certain actions will, by the law of kamma, be followed eventually and inevitably (17) by a pleasurable experience, vipaka as karmic result.
Our discussion above, however, suggests that by far the more usual meaning of the term, alluded to but not explicitly recognized by the authors, is a force of goodness, generated through certain actions and accumulated until expended in actually experienced vipaka as karmic result.