Vinistes, Vistes, Vencistes

I found this on Museo de los horrores through el Foro Cervantes via La Palabra Del Día from elcastellano.org. This is one of my biggest pet-peeves, and naturally so as I am quite the grammar nut.

*Dijistes, *vinistes

In spoken language, although it is considered a vernacular trait, the use of a final -s on the singular second person of the indefinite preterit (or simple perfect) has been expanding. Many of our visitors to el Museo point it out to us in their letters; but the worst part is that it is beginning to be reflected in the written language as well--especially in the press.

The indefinite preterit is a verbal tense of the indicative mood, that expresses an action in the past in regards to that in which the speaker is, whose end has been consummated. But what we care about now is the paradigm, with a very common example: the verb decir.

Yo dije
Tú dijiste
Él dijo
Nosotros dijimos
Vosotros dijisteis
Ellos dijeron

But by analogy with the rest of the verbal tenses (dices, decías, dirás...), as vernacular a final -s is added to the second person (tú), and so we find the vernacular:

Tú *dijistes

In the southern norm of Spanish and in American linguistic variations, this problem is also found.

Vos *dijistes