What can I use as a vinegar replacement?

You can use lemon juice in place of vinegar, using double the amount of vinegar required. 

If you need a non-acidic alternative, you can use 3/4 tsp of cream of tartar and 2 Tbsp of water. Combine the cream of tartar with the dry mix and blend well. In a separate bowl, combine the eggs, water and oil (remember to add the water instructed on the box, plus the 2 additional Tbsp to replace the liquid of the vinegar). Combine the dry and wet ingredients and continue as instructed on the box.