Is there a difference between yatha bhuta and vipassana? Or are they just synonyms?
The two terms are not exactly synonymous although their context of usage is synonymous since both terms are associated with "seeing" ("passa") the same true reality.
'Yathā bhūta' means "true nature", as is found in AN 11.2, namely: 'yathā (true) bhūta (nature) jānāmi (know) passāmi (see)*'
"Vipassana" means "clear seeing" (namely, the Four Noble Truths, the true nature of suffering & peace; & the Three Characteristics, the true nature of conditioned things).
Vipassana is the process of methodology to see things as they are. Seeing this as they are (yathā,bhuta.ñana.dassana) leads to revulsion (nibbidā) --> dispassion (virāga) --> liberation (vimutti) --> knowledge of the destruction of the influxes (āsava-k,khaya,ñāna). [Upanisa Sutta, (Ekā,dasaka) Cetanā’karaṇīya Sutta, (Dasaka) Cetanā’karaṇīya Sutta].
So by means of Vipassana you get yathā,bhuta.ñana.dassana though which you can get liberation transitively. Vimutt’āyatana Sutta mentions this, though it skips some intermidiate steps Nibbida fomular implies that you have to pass through yathā,bhuta.ñana.dassana. Also see this answer.
Also see: Nibbida by Piya Tan
"yathabhuta - seeing things as they are - not as you want them to be" - this was the instruction at Goenka retreats. It relates to the four noble truths - suffering is because we want things to be different than they actually are in reality...in short - craving is what suffering is...