I remember you posing the question to me prevously, and I just read the relevant portions in "A History of the Ancient Near East" to see if I could get some more clarity in the matter, and it appears that the matter is, simply put, poorly understood. The only known relative of Urartian is Hurrian, but it is apparently not a direct descendant (Armenian, meanwhile, is Indo-European). Urartian is known due to dual inscriptions in Assyrian and Urartian. Interestingly, the earliest inscriptions by the Urartu seem to have been in Assyrian. This suggests at least to me that if there was a prestige language among the elite, it was Assyrian, rather than Urartian, which makes perfect sense given the politics of the region, and given the earlier presence of Hurrian kingdoms, it would seem that Hurrian-Urartian languages were the vernacular of the region.

The work simply notes that, in the 6th century, Armenia had certainly replaced Urartu. Since the Urartian kingdom originated around Lake Van, and the Hurrian kingdoms had lain south of it, it would seem plausible that Armenians were subjected during its expansion. But the Armenian language is notoriously difficult to classify, making it hard to understand Armenian ethnogenesis. In some ways it seems to occupy the space that divides the Graecian and Indo-Iranian languages (Being a Satem language like Indo-Iranian, but sharing other characteristics with Greek). Especially given that the collapse of Urartu appears to have been at least partly due to northern or northwestern pressure, it does not seem impossible that the emergence of Armenia might have correlated with the migration of Armenians into the region from the west or north, around the coasts of the Black Sea. But there appears to be no strong evidence either way.

In either case, Urartian does appear to have been a vernacular, but not strongly linked to the Armenian language.