Hey Charlie, Ill give an outline to my understanding of the differences between these meditation techniques- based on my experience purely. The basic difference between the technique of zazen and vipassana is that zazen, or zen, emphasizes mindfulness- observing the mind. zazen will use an object for concentration, such as counting breaths, a visual aid, a mantra or some puzzle. Mindfully engaging in daily activity is also very important. The final aim is to observe and remain in the the present moment of the mind, without thinking and reacting with desire or aversion. Vipassana also aims at remaining in the present moment without reacting with aversion or desire. However, this is achieved by observing the reality of the body without any outside object for aiding concentration. Also, understanding of the reality of the mind comes with observing the reality of the body and its sensations. Mindfully engaging with the mind is thus not stressed in Vipassana as a mindful mind is considered the natural byproduct of understanding that comes by observing the wealthy reality of the body. The reason for concentrating on the sensations of the body is that desire and aversion are not just habits of the mind but actually reactions to sensations in the body. The understanding that sensations in the body just happen, just like life just happens, is the next step I referred to before. If we want to become freed, we must change the way we react to these sensations. And for this, we need to become aware of the sensations.
Im lucky to have had first hand experience of all this at the course and I encourage anyone to also take the time for themselves and attend a course. All the work you do there is essentially up to you, the teacher is useful in guiding you to do Vipassana and not become confused or entangled in other techniques. As said earlier, there are many different ways of meditating, all with their own aims. The aim of Vipassana is to reveal the root of desire and aversion which, according to the Buddha, are the reason for our unhappiness. Why this is important is a wide subject itself. If anyone is interested, they should read more about Buddhism. However, a practical experience of his teachings, such as gained from Vipassana, is much more useful.