For most powders (depending on the composition/ingredients) you use it in the same ratio as the liquid extract. Basically one rounded teaspoon of powder = 1 teaspoon of liquid extract.
Who makes the powder and what are the ingredients? Three are many types of vanilla powder, and most are synthetic. Some are a sugar base sprayed with vanilla extract, some are ground vanilla beans mixed with sugar, some are pure ground vanilla beans. What color is it? If it is black, then it is ground vanilla beans and very potent. You should only use half of what you would use with an extract. If it is white, then it could be sucrose and vanilla extract, or it could be artificial vanillin (synthetic), and is probably Crystalline Phenolic Aldehyde. If it is brownish, then it is probably dextrose with vanilla extract.
So, can you check the ingredients and list them along with the vendor name? Also, remember that vanilla is strong! More than a teaspoon can be overpowering and bitter.