Most brownie recipes call for vanilla. Will the earth stop turning if you omit it? No, but anyone with active taste buds will notice a difference.

The basic brownie recipe in Joy of Cooking (Canada, UK) calls for one teaspoon of vanilla, as do both the cake- and fudge-type brownies in the Better Homes & Gardens New Cook Book (Canada, UK). The Fannie Farmer Cookbook’s (Canada, UK) recipe calls for two tablespoons, though, which is a lot of flavor and a lot of liquid to leave out.

The fancy Pierre Hermé Moist and Nutty Brownies recipe found here at Ochef, however, does not require vanilla, so you may be off the hook.

the same time, vanilla lasts a long time - you really should keep some on hand for moments like this.