Why Vipassana meditation need a base object? I think (please correct me if I am wrong) that Vipassana is about letting go of control and judgement, and the meditator will just become aware and accept things as they are. Be it pain, thoughts, sound, or physical sensations like breath or a moving stomach (as we breath).

I wonder we need to have a base object, like breath or moving stomach. Why don't we just let go from the very beginning and just become aware and accept any sensation as it is, without any default object?
I think (please kindly correct me if i wrong) Vipassana is about letting go of control &amp; judgement, and the meditator will just aware and accept things as it is. Will it be pain, thoughts, sound, and physical sensations like breath or a moving stomach (as we breath).


That sounds more like shikantaza/zazen.

Vipassanā means insight into reality: seeing the three marks of existence dukkha, anicca, anatta. The practice is described in suttas such as Satipatthana and Anapanasati. So, the actual goal is not "letting go of control &amp; judement" -- however, "letting go" is a useful tip at some points in the practice.


  I wonder why do we have a base object, like breath or moving stomach. Why dont we just let go from the very beginning and just aware and accept any sensation as it is, without any default object?


Because insight into reality need some faculties developed. Two of them are concentration (samadhi) and mindfulness (sati). It so happens that is very difficult to develop concentration without something to anchor our attention: meditation objects.

Finally, samadhi and sati are not just random faculties. Both are listed in the Noble Eightfold Path and in the Seven Factors of Enlightenment.