Yes, Vaishya is mentioned in all the 4 Vedas, because the Purusha Sukta has the word in it. And, the Purusha Sukta is found in all the 4 Vedas.
BrAhmano asya mukham Asit
BAhu rAjanyah krtah
Uru tad asya yad
PAdbhyAm shudro ajAyata ||
His key part (or mouth) was the man of the word (BrAhmana). Into the
Prince (rAjanya), (referring to Kshatriyas) his arms were made. While
his wideness (Uru;Thighs) became the producers (Vaishya). His power of
movement (feet) gave birth to the man of service (Shudra).
Rig Veda 10.90.12 Or VAjasaneyi SamhitA 31.11.
Another Mantra from Yajurveda that mentions Vaishya is the following:
Rucham no dehi brAhmaneshu (1),
Rucham rAjasu nah kridhi (2)
Rucham vishyeshu shudreshu (3)
Mayi dehi ruchA rucham (4)
Give lustre to our BrAhmana-s (1), Give lustre to our kingly men (2).
Give lustre to our Vaishya-s and Shudra-s (3). Give lustre to me (4).
Shukla Yajurveda 18.48
Location of the Purusha Sukta in the different Vedas are as follows:
In Rig Veda it is 10.90, in Shukla Yajur Veda's Vajasaneya Samhita it is the 31'st Adhyaya. It is not directly found in Krishna Yajur Veda Taittiriya Samhita (KYTS), but it is in the Taittiriya Aranyaka (TA) which is in fact a continuation of KYTS. And, here it is TA 3.12. The Purusha Sukta of the Sama Veda has only 6 Mantras.