a few things..
vijjavimutti - 'clear knowing and release' as translated here ' -vijja is the opposite of avijja- hence wisdom or insight might be a better translation IMO, as avijja is delusion and clear knowing maybe confused with clear comprehension (sampajanna).
Note how in and out breaths are felt with body, feelings, mind and insight. The Buddha is clearly not stuck with the Two Truths version of reality (ultimate truth and conventional truth) and doesn't make it a hindrance to progressing further. The mindfulness used here is clearly more broader and less intense, to incorporate these other elements along with the breath- and uses 'conventional' reality. To get to ultimate reality the yogi would have to use highly magnified mindfulness noting fine extremely fine details. This method is highly commendable, but not absolutely necessary by the looks of it. If there is anything which can lead to revulsion, dispassion and cessation (foulness for example, using visualisation, but nevertheless a way to access the truth) then it is a way forward.
Yoniso manasikara- appropriate contemplation/attention, the factor the buddha described as the most useful internal thing in attaining nibbana (see udana), is a verbal pointing out of the truth to oneself, leading to the truths being seen (dassana) in the present moment and leading to dispassion and cessation (see 4 sotapatti anga). Note below that thoughts like 'a cancer an affliction' cannot arise on their own in a prolonged repeated way, unless we intentionally think of them. They are simply a way of 'intensifying' the truth and cutting through avijja. Yonismanasikara becomes an internal spiritual friend who points to the truth. Yoniso manasikara methods are also conventional truth methods.
"A virtuous monk, Kotthita my friend, should attend in an appropriate way (yoniso manasikara) to the five clinging-aggregates as inconstant, stressful, a disease, a cancer, an arrow, painful, an affliction, alien, a dissolution, an emptiness, not-self. Which five? Form as a clinging-aggregate, feeling... perception... fabrications... consciousness as a clinging-aggregate. A virtuous monk should attend in an appropriate way to these five clinging-aggregates as inconstant, stressful, a disease, a cancer, an arrow, painful, an affliction, alien, a dissolution, an emptiness, not-self. For it is possible that a virtuous monk, attending in an appropriate way to these five clinging-aggregates as inconstant... not-self, would realize the fruit of stream-entry."
C:\Documents and Settings\Matheesha\My Documents\important suttas\yoniso manasikara\SN 22_122 Silavant Sutta.mht