It can be plural or singular, "vates", but only in the nominative.
Vates sacrificant = The soothsayers sacrifice.
Vates sacrificat = The soothsayer sacrifices.
In the accusative, it's Vatem (or even "Vaten" says my dictionary) for singular, and Vates for the plural.
Templum illud habet vates = Several soothsayers.
Templum illud habet vatem = 1 soothsayer.