If Sampajanna is clear comprehension and Vipassana is clearly seeing how or are they distinguishable?
Sampajanna is a prerequisite for right concentration. Vipassana is a result of right concentration.
Mindfulness > sampajanna > concentration > vipassana > liberation > nibbana
Sampajanna is a product of active volition & mindfulness; what is called 'wise or careful attention' (yonisomanasikāro); as described in AN 10.61:
Mindfulness and clear comprehension, too, I say, have a nutriment; they are not without nutriment. And what is the nutriment for mindfulness and clear comprehension? It should be said: careful attention (yonisomanasikāro). AN 10.61
Vipassana is the automatic seeing of ultimate truth that occurs when the mind is clear & concentrated, as described in AN 11.2.
For a person whose mind is concentrated, there is no need for an act of will, 'May I know & see things as they actually are.' It is in the nature of things that a person whose mind is concentrated knows & sees things as they actually are. AN 11.2
Sampajanna (applied wisdom) is the preliminary wisdom that mindfulness (remembering; recollection) brings to mind to establish meditation. Again, to quote AN 10.61:
The four establishments of mindfulness, too, I say, have a nutriment; they are not without nutriment. And what is the nutriment for the four establishments of mindfulness? It should be said: ... mindfulness and clear comprehension. AN 10.61
For example, when meditation is established, sampajanna applies the types of wisdoms listed below, when required:
observing breathing brings peace therefore it is beneficial to observe the breathing
craving & attachment cause suffering & disturbances therefore I should meditate without craving & attachment
this hindrance harms myself & harms another therefore it should be abandoned
Bhikkhu Buddhadasa has explained 'sampajanna' & 'vipassana' below:
sampajanna, wisdom-in-action, ready comprehension, clear comprehension: the specific application of panna as required in a given situation.
vipassana, insight: literally, "clear seeing," to see clearly, distinctly, directly into the true nature of things, into aniccam-dukkham-anatta. Vipassana is popularly used for mental development practiced for the sake of true insight. In such cases, the physical posture, theory, and method of such practices must not be confused with true realization of impermanence, unsatisfactoriness, and not-self. Vipassana cannot be taught.
Sampajanna is wisdom as it meets up with and immediately confronts a problem, as it deals with and wipes out that problem -- this is wisdom-in-action. It is only that wisdom specifically related and applied to a particular situation or event. The word "wisdom" (paññä) encompasses many meanings and understandings, we can't even begin to estimate its content. However, the word "sampajanna" is far more limited in its meaning. It is exactly that wisdom directly needed for the problem that confronts us. Active wisdom isn't general, it is a matter of particulars.
We can compare wisdom (paññä) with the medicine chest in our house. In it we store a wide variety of drugs, pills, capsules, ointments, powders, and syrups for possible use. When we're actually sick, we must choose from among the many the one drug which will be effective in treating our ailment. We can't take them all; we take just what is needed to cure our illness here and now. The same is true for wisdom. Understand that there's an incredible amount of what we call paññä, but that we only apply a little bit at a time. We apply just that portion which can take care of the immediate situation. Know how to use the Dhamma, the paññä, which is exactly relevant to our situation and problem. The Dhamma or wisdom which controls that situation and problem is what we call "sampajanna."
In sampajaññaṃ 4 --asammohasampajañña is vipassana.
sampajaññaṃ 4 = clear comprehension; clarity of consciousness; awareness
- sātthakasampajaññaṃ - clear comprehension of purpose.
- sappāyasampajaññaṃ - clear comprehension of suitability.
- gocarasampajaññaṃ - clear comprehension of the domain in physical and mind.
- asammohasampajañña - clear comprehension of non-delusion, or of reality.
From this it becomes evident that according to the Buddha, whenever there is sammasati or satipatthana, it is always with sampajanna. That means it is with panna (wisdom). Otherwise it is mere sati, which is mere remembrance or awareness.