U ba Khin / Goenka Vipassana putting stress only on vednanupassana The Vipassana as taught by Gautam Buddha in his Maha Sati Patthana Sutta talks about  four different feilds of meditation : Body, Sensations,mind and mental contents.

Goenka focuses solely on sensations - Isthis not an incorrect application of Buddhas teaching. If focussing on only one frame of reference could liberate the bhikku, why then would he have specifically mentioned four?
'Sensations' of the body are generally not 'vedana' (feelings). Instead, they are generally mental formations (sankhara) of stress stored in the physical body, as follows:


  The craving that makes for further becoming —
  accompanied by passion &amp; delight, relishing now this &amp; now that —
  grows within him. His bodily disturbances &amp; mental disturbances grow.
  His bodily torments &amp; mental torments grow. His bodily distresses &amp;
  mental distresses grow. He is sensitive to bodily stress &amp; mental
  stress. MN 149


'Feelings' ('vedana') refer to the pleasantness &amp; unpleasantness that is associated with sense contact via the eyes, ears, nose, tongue, body &amp; mind. 


  There are these three kinds of feeling: a pleasant feeling, a painful feeling and neither-pleasant-nor-painful feeling. MN 74
  
  And what is feeling? These six are classes of feeling: feeling born from eye-contact, feeling born from ear-contact, feeling born from
  nose-contact, feeling born from tongue-contact, feeling born from
  body-contact, feeling born from mind-contact. This is called
  feeling. SN 12.2


Observing only genuine vedana (feeling) can liberate a bhikkhu, per the Culatanhasankhaya Sutta. 


  Here, ruler of gods, a bhikkhu has heard that nothing is worth adhering to. When a bhikkhu has heard that nothing is worth adhering
  to, he directly knows everything; having directly known everything, he
  fully understands everything; having directly known everything, he
  fully understood everything, whatever feeling he feels, whether
  pleasant or painful or neither pleasant or painful, he abides
  contemplating (observing) impermanence in those feelings,
  contemplating (observing) fading away, contemplating (observing)
  cessation, contemplating (observing) relinquishment (letting go).
  Contemplating (observing) thus, he does not cling to anything in the
  world. When he does not cling, he is not agitated, he personally
  attains Nibbana. He understands: ‘Birth is destroyed, the holy life
  has been lived, there is no more coming to any state of being.’
  Briefly, it is in this way, ruler of gods, that a bhikkhu is liberated
  in the destruction of craving, one who has reached the ultimate end,
  the ultimate security from bondage, the ultimate holy life, the
  ultimate goal, one who is foremost among gods and humans. MN 37