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
Yatha does not mean "true." Butha does not mean "nature." Yatha means "as" or "like." Butha means either "[it] is" or "has come to be." Yatha butha therefore means "as it (really) is," or "as it has come to be." There is an implication of truth/reality, but that is not literally present in the language.
Vipassana is neither process or method as you can use multiple methods to attain vipassana meaning "insight" (lit. vi = "in" or "into" [in this context], passa = "see," na = nominalization marker).
Therefore cultivating samadhi allows us achieve vipassana regarding reality yatha butha. Though, the causality could be reversed and you could say that when the mind is still in samadhi then we are able to see experience yatha butha, which then becomes the cause for vipassana.