hello, newbie baker here.. i have a vanilla cake recipe that i like, and i want to try and turn it into an orange cake.. i'm planning to add the zest of 1 orange and substitute some of the milk with the juice of that 1 orange, can i do that? the recipe only uses baking powder, do i need to add baking soda? if yes, how many baking soda do i need? thanks in advance for the help..

Put the orange juice onto the measuring cup and then add milk to the required mark.

I would not adjust the baking powder or add soda.  But when you add fruit juice you need to put the cake batter into the pans promptly.  So get the pans ready before you start mixing.

When I use my vanilla cake recipe to make orange cake (or lemon or lime) I zest 2 oranges (a sweet variety like navel oranges--I actually smell them before I buy because the stronger the scent, the better the flavor of the cake) the night before and add the zest to the measure of sugar called for in the recipe.  I store the sugar and zest mix overnite in a ziplock bag.  The next day I proceed as normal with the recipe.  I squeeze the oranges that I zested the night before and use up to 1/2 cup of juice to replace part of the liquid.  I don't add baking soda at all, and mine turns out with the same texture and crumb as the straight up vanilla.  The resulting cake is intensly orange (or lemon or lime) flavored.

I'd just do a test run to see if your vanilla recipe will hold up to the changes. hth