Should I mix vanilla sugar with wet or dry ingredients? I could not find vanilla extract in my town. There is vanilla sugar instead in markets.In my cake recipe, there is  vanilla extract and it is whipped with egg, sugar and oil. But I am not sure with vanilla sugar.

On my first attempt I mixed it with dry ingredients, but the cake I made had too much crystallization. I wasn't sure if that was the reason.

Should I mix it with my dry ingredients like flour, baking powder and etc or with egg and sugar in the first step. ? 

Thanks.
You should use it as the sugar the recipe calls for.  In this case, whip it with the eggs and oil.  The vanilla in vanilla sugar is for flavoring and does not really alter the sugar.