The DC for Poison is a formula. The static value listed in a monster's stat block is a convenience so that GMs don't have to calculate it themselves.
From the d20 SRD:
The Fortitude save DC against a creature’s natural poison attack is equal to 10 + ½ poisoning creature’s racial HD + poisoning creature’s Con modifier (the exact DC is given in the creature’s descriptive text).
So if you increase the viper's Con modifier (by spell, feat, item, etc.) the DC for the venom will increase. Off the top of my head Bear's Endurance or an Amulet of Health would both be helpful in increasing the venom's DC.
You can also have the viper take the Ability Focus feat out of the monster manual which ups the DC of a chosen special attack by 2.