How to change a chocolate cake recipe to a vanilla cake
My honest answer is you don't without a lot of experimentation. 1.5 cups of cocoa powder is a significant portion of the recipe, taking it out will throw out every other measurement and there is no easy way to calculate what adjustments you'd need to make. Vanilla powder is much stronger a flavor than cocoa powder and cannot be a direct substitute. 

If you take out the cocoa powder you have to reduce the sugar level, but removing that much dry ingredient will make your cake too moist so you'd need to reduce wet ingredients to compensate. The buttermilk adds a sourness to it that counterbalances the bitterness of the chocolate, so that would need to be factored in. By the time you consider all that there are too many factors to make it worthwhile, you're better off finding a vanilla cake recipe. There are plenty out there which have buttermilk in them (I like buttermilk so I'd give that a try), I've had good luck with recipes using custard powder as well.