This Latin phrase from Lonesome Dove is neither nonsense nor bad Latin.  It means “A grape becomes mottled by being a grape,” with the sense: it is the nature of a grape to become “mottled” as it ripens.  In the context of the novel, this means, “Character is destiny” — the truth of which is illustrated time and again in the novel. The repetition of the ‘v’ strengthens the point. It is lovely Latin.

vivendo here is a gerund of vivo, in the ablative (instrumental). 

uvam is a cognate (a play on vitam vivendo) or internal accusative, a not uncommon feature of ancient Latin.

Just as a grape or a man can become nothing other than what each is, pigs remain true to themselves. “We don’t rent pigs. . .  Goats and donkeys are neither bought nor sold,” only cattle and horses whose character man has fundamentally changed.