Not to state the obvious, but unless you’re an internationally successful cat bugler, it wouldn’t matter if the store were 12 short feet away if it’s closed. The vanilla is added to your recipe only for the delicious, indescribable, je-ne-sais-quoi flavor that it will impart, not as an essential bit of extra moisture. So you can just leave it out entirely and imagine what your bread would have tasted like if you had planned ahead or gone to cat-burgler school.

There are other extracts that can sometimes be substituted for vanilla – almond extract is one choice. Rum extract is sometimes used. Chocolate extract is an option. As mentioned in another Ochef Q&A, there is an Italian product called Fiori di Sicilia, which can stand in for vanilla sometimes. But we can’t really think of a flavoring that would definitely improve your banana bread, so why risk messing it up with an experimental ingredient? It will still be delicious.