A common question about vanilla extract is: What is the best alcohol for vanilla extract?
A 1995 study showed that 10% more vanillin was extracted at 47.5% ethanol than 95% ethanol [page 16, pdf!].
Vodka is the alcohol of choice because it has a neutral flavor. Other liquors can be used, but they contribute flavors of their own. Commercial extracts use a neutral flavored grain alcohol (vodka), but you are free to use rum, brandy, gin, whatever.
Coco&Me just started an extract experiment with different types of alcohol.