As far as I understand, Vipassana meditation trains attention and equanimity. Which makes sense because you are basically doing body scans all the time and remaining equanimous to bodily sensations, whether they are pleasant or unpleasant. Cool. But ... how can you develop love, compassion and sympathetic joy by doing that? How can "watching equanimously your bodily sensations" make you a more loving and compassionate person? At best you should become a very neutral, equanimous, cold person with a very high level of awareness, but then how would you cultivate sympathetic joy, loving kindness, compassion? Does one need to complement vipassana with other techniques to develop those qualities?
Your concern is based on invalid definition of Vipassana/Vipashyana. "Watching equanimously your bodily sensations" is not what Buddha meant by insight meditation. Insight meditation is rigorous introspection of one's mental processes seeking answers to questions like:
- What is it that we call "I" and how can it be identified exactly in our subjective experience? Who or what is the subject of experience that does the watching?
- What are the limits to the extent that we can be said to have free-will and to have an identity?
- How do the trains of thought work?
- What exactly happens when the mind engages in relative judgement?
- How does the mind create its notions of entities?
- How is reality we perceive relate to the mind perceiving it?
- How do emotions work and what do they depend on?
- What are preconceptions and attachments and why they are problematic?
- What are overgeneralization and reification and why are they problematic?
- Why exactly do we feel unhappy?
- How do psychosomatic experiences relate to our subconscious mind and to our emotions?
- How is emotional energy drained and how is it generated?
- What can we change in our behavior in light of all the factors above, in order to be stronger and happier?
Now, this is vipashyana - sitting and looking at one's mindstream in order to get answers to these questions.
Once you get some answers to most of the above - and with your new perspective you look back at the state of humanity -- you suddenly find yourself having the very same loving kindness, compassion, and sympathetic joy arising quite spontaneously ;)
Well-developed Buddhist samatha-vipassana meditation cleanses the body & mind from mental impurities such as greed, lust, hatred, anger, delusion, etc. Also, to practise samatha-vipassana successfully requires lots of self-love or self-compassion. Thus, when the true results of samatha-vipassana meditation are developed, what will remain is bliss & love. In summary, when unwholesome emotions are cleansed, what naturally will remain are wholesome emotions.
Therefore, one does not really need to complement samatha-vipassana with other techniques to develop those qualities. Often, sympathetic joy, loving kindness, compassion, etc, need to be developed when there are hindrances to samatha-vipassana. But if there are no hindrances to concentration, those qualities will be included in the natural results of samatha-vipassana.