So I pulled some stats up.
From the start of 1976 to the end of the 1980/81 season Viv averaged 72.56 at Test level, and that's not even considering his WSC record. That is genuinely fantastic and would be beyond a mere one year wonder.
Between 1981/82 and 1988/89 he averaged 44.26, which admittedly is.....kinda meh by his reputation? It's still good, especially for the era he played in (though he did have the fortune of never facing the best attack in the game), but it's not as stellar as it should've been. I guess this was where the intangible ratings come into it? Feels very Pietersen post-captaincy esque.
The back end of his career he was definitely past it (
averaging 36.36 after the 88/89 season), but he does have the excuse of being in his late 30's by that point.
EDIT: One test should've been in the first than the second category, with that taken into account he averages 71.38 in the first period and 44.59 in the second.