I imagine you are asking about a vanilla powder such as this one from Nielsen-Massey marketed by King Arthur Flour which is vanilla and maltrodextrin, or this one this one, marketed through Amazon claims to be stronger than vanilla extract, and to consist of "vanilla bean extractives [sic], evaporated cane juice, silica, cellulose."
If these are typical products, most of what is in the bottle is filler—and the rest is vanilla flavoring. Both of these particular products indicate that their flavoring is natural, although it almost certainly is created by making vanilla extract, then evaporating the solvent, much as instant coffee is made.
Of course, most of what is in a bottle regular vanilla is alcohol, water, or sugar depending on the specific brand. Only a bit is actual flavorants from vanilla beans.
So the real issues become:
- What is the relative strength of vanilla flavoring on a measure per measure basis
- How do the carriers affect a given recipe
I cannot answer the first question—hopefully someone else can provide insight there, but it will probably vary by brand or specific product.
The second property opens up new opportunities for the powders:
- Since they have no water, they can be added to chocolate without causing seizing
- They can be used in dry mixes, such as a homemade hot chocolate mix or pancake mix
- They can be used in coatings or powders, as for powdered donuts
- They have no alcohol, which may or may not make them acceptable to those who avoid all alcohol for religious reason (I am not expert enough to say this as an absolute, because it is likely alcohol was used in their manufacturer to create the extract used to make the powder)
In most typical applications, you should be able to use one of the powdered vanillas. However, I cannot tell you the ratio of substitution—hopefully your specific product has guidance on its packaging.
The one place I would not try it is a delicate icing (in the case of the second product) as some of the fillers may give it a gritty texture.