In vipassana practice, we become aware of our ever-changing experiences, without adding to what is going on through our reactions and projections. In metta practice, we direct lovingkindness toward ourselves and then, in a sequence of expansion, towards somebody we love already, somebody we are neutral towards, somebody we have difficulty with, and ultimately toward all beings everywhere without distinction. The main difference between metta and vipassana is that metta is a concentration practice, while vipassana is an insight practice. This is a functional difference. If you're doing mindfulness practice, there is no such thing as a distraction. You pay attention to whatever arises in your awareness and make that an object of meditation. There is no sense of preferring one experience over another, since each experience is seen as having the same ultimate nature. Each is characterized by impermanence (anicca), unsatisfactoriness (dukkha) and having no separate existence (anatta). You can see these characteristics by looking at either pleasure or pain.
In contrast to vipassana, in metta practice you are not focusing on the ultimate nature of phenomena. Furthermore, you are choosing a particular object of meditation, which is the metta phrase, such as "May I be happy" or "May I be peaceful." You hold the phrase in your heart just the way you'd hold something fragile and precious in your hand. As you cherish each phrase, distractions inevitably arise. Your head starts itching or your knee starts hurting or you start thinking about the phone call you didn't make. When you get distracted, you drop the distractions as quickly as possible and come back to the phrase, the chosen object of meditation. Choosing a particular object to stay focused on makes metta a concentration practice. When some other experience arises you don't explore it, note it, or try to see its changing nature.
Nonetheless, I still call metta "a sneaky wisdom practice," because people often have enormous insight doing metta.
In metta practice people are amazed to find out that they have a capacity for lovingkindness, both for themselves and for others. Due to our past conditioning, many of us do not trust our capacity to love. Metta involves a tremendous opening and purifying of our fields of intention, which can then infuse our vipassana practice as well as our entire life. We discover that we can indeed love and that everything comes back to love.