Both Anapansati and Vipassana are described as the direct path to Nibbana. What are the main differences along these paths?
Does one fully include the other, or do they diverge from each other, or they in a sense parallel to each?
Anapana is Vipassana Meditation also.
It has 16 stages. In sets of 4 (triads) they cover the 4 Satipatana.
If I am to reinterpret the question above as "What is the difference between Anapansati and Satipatthana" as I feel this might be what you meant.
Satipatthana shares as section on Anapana. They come up to "Sabbakayapatisamvedi" - you are sensitive to your whole body; "Passambhayam kayasamkharam" - calming the body fabrication. (Disclaimer: this might be interpreted differently by different linages / teachers.)
In Anapana you more on to feelings which has arisen (when you have calmed your bodily fabrications and you are sensitive on your whole body generally it is pleasant)
In Satipatthana stay with the body, looking at it in more detail and remedying general miss perceptions about it if you have such polarity. When you are doing this you are looking at the feeling outline of the body as initially developed you sensitivity so you can feel the full outline of the body.
Though at the very high level the flow of both methods are the same. Body <=> feeling (**) <=> mind <=> dhamma.
Also any Buddhist Meditation you start off with you have to come to the junction of Sabbakayapatisamvedi and as you progress it will not be limited to the Satipatthana you started off with. In Anapana you start off with the breath but all 4 Satipatthana develops in the process. All 4 has to develop as the Satipatthana correspond to the aggregates. Also any impact on one aggregate effects the others. Hence something you observe in one foundation of mindfullness will impact the other.