Since vos came from a different form of the verb than tú it is not surprising to see that the vos form of the verb often (although not always) uses a different form of the verb than the tú.

If you remember your vosotros endings of the present tense you will assume that the endings evolve to "áis" "éis" and "ís", however the most common system of vos endings in the present tense is the following.
 
 

Stem changes such as o to ue, e to ie, e to i, do not occur. For certain one syllable verbs and estar, there is no difference between the tú form and the vos form (since one syllable words do not usually take accents).

For example, compare the following forms:
 

However, for most verbs there is a difference:
 

In most other tenses the verb forms are the same, except for the affirmative vos command.  That form is simply the infinitive without the r and with the vowel of the infinitve ending stressed with a written accent if it's more than one syllable or would otherwise need an accent.