It seems like Abhdihamma refers to citta as the essential knowing nature of the mind. The purest moment of cognition or mind. Other stages have been given names too (see below). Those are progressively defiled.
The citta just knows. In this context vinnana and vinnana-khandha refer to defiled stages of cognition.
In a simile, citta is the mirror-like mind. It just reflects (knows) and gets not involved via anusaya, asavan and tanha in the reflections. Vinnana does or has.
Citta is just mirror-like. It does not want anything. It has no like and dislike. It has no tendency. It has no longings, it has no inclination, it signless. It just reflects or knows.
All the five khandha’s are aspects of the mind or cognition and always present in cognition.
Vinnana is in this context often seen as just the first moment of cognizing an odour, sound etc. but that cognizing is so quickly defiled with anusaya, asava, tanha, that it is useful to distinguish it from citta.
The cognition of an arahant and Buddha does not defile anymore to vinnana stage.
The nine stages are: citta, mano, mānasaṃ, hadayaṃ paṇḍaraṃ, mano manāyatanaṃ, manindriyaṃ, viññāṇaṃ, viññāṇakkhandho.
Anyway: What needs attention, i think, is that just sense-vinnana almost never exist for us, because it is immediately contaminated with anusaya, asava, tanha. So, in reality vinnana is much more than only the six sense consciousnesses It is more a mix of sense-objects with longings, perceptions, emotions, views, expactations, signs, tendencies etc. That is all incorprated in the vinnana.
In this context it is useful to distinguish this from the citta in which this is not going on.