The term “Reductionism” has been employed by Herbert V. Guenther to render the Tibetan term dngos por smra ba (spelled wrongly there as dngos smar ba), that is, Vastuvāda. This philosophy is said to include “what seems to be a kind of naive realism through representative realism towards subjective idealism” (of Vaibhāṣikas, Sautrāntikas, and Vijñānavādins, respectively). See Guenther 1989: 162 “This term may best be translated by ‘reductionism because each system so far mentioned tries to reduce the whole of reality to one or more particular existents (dngos-po).”
But I wonder if this term is useful at all in conveying the ontology posited by those Buddhist schools. What is not made clear there is that the designation Vastuvāda or Vastuvādin is not a term that the proponents of those schools employ but has been (if I am not mistaken) given (not quite flatteringly if not pejoratively) by their Mādhyamika opponents. Some Tibetan schools have gone to the extent of claiming that a Vastuvāda view is not in a position to independently cause the soteriological breakthrough.
What is, however, true is that Vaibhāṣikas, Sautrāntikas, and Vijñānavādins are said to posit some kind of entities (vastu: dngos po), be they be cognitive (shes pa) or inanimate (bem po), to be existent, not just nominally (prajñaptisat: btags par yod pa) but substantially (dravyasat: rdzas su yod pa) and hence they are labelled as Vastuvādins (dngos por smra ba) and also as Sasvabhāvavādins (ngo bo nyid yod par smra ba) as opposed to Niḥsvabhāvavādins (ngo bo nyid med par smra ba).
What is also true is that all Buddhist schools reject any metaphysical substrate or cause (e.g. God or Self) of the phenomena and each school reduces all existents to “x-mātra.” By understanding the value of “x” that each school accepts and the value of “y” that it rejects, we may understand the kind of ontology proposed by each system. Nonetheless, the term “reductionism” here seems simply unserviceable.