According to everything that I've read about "vos" and its variations, the standard, historical conjugation pattern was as follows: Take the "vosotros" (2nd-person familiar plural pronoun; its historical counterpart whose connection has long been broken), and remove the diphthong if there is one. Here are some examples:
In the present indicative:
tú sabes ... vos sabés ... vosotros sabéis
tú dices ... vos decís .... vosotros decís
tú comes ... vos comés ... vosotros coméis
In the preterite (simple past):
tú dormiste ... vos dormistes ... vosotros dormisteis
tú comiste ... vos comistes ... vosotros comisteis
In the present subjunctive:
tú comas ... vos comás ... vosotros comáis
tú digas ... vos digás ... vosotros digáis
In the present perfect:
tú has comido ... vos habés comido ... vosotros habéis comido
Now, it would be swell if everything worked out like I wanted it to, but Argentina is giving me quite an exercise in the "adaptability" and "patience" sectors of my brain. What's interesting from a descriptive point of view is that, although "vos" is used exclusively as a 2nd-person familiar pronoun here, the conjugation only follows my expected pattern in the present indicative. Of all 14 tenses and moods in Spanish, only one fits the pattern. Otherwise, the Argentine "vos" behaves oddly like two. It's just in the most common of tenses that we hear that sassy vos and get the resulting shift in emphasis.
Of course, it's possible that what I learned about voseo is totally wrong, too.
Food for thought: eat it up.