According to Bhikkhu Bodhi, some Citta Vitthi do not even go to the stage of “Pancadvara citta”, e.g. the very slight object at page 161. But Bhikkhu Bodhi also calls the” Vibrational Bhavanga citta” a resultant, which I translate as vipaka?
The other point:
If you say it is not a Vipaka, then your statement:” I think it is not only possible but necesssary. For example, at the cuti-patisandhi moment, a vipaka bring the next bhava can come through any of the six senses. How does an arupavacara Brahma grasp the next bhava at the cuti-patisandhi moment? He has only the mind-door.”
It would mean the dhamma that initiates the cuti-patisandhi transition in arupa-realm is not a vipaka?