i am doing a reversible vest and i know that i got to sew it inside out first but were do you leave a hole to turn it inside out

Usually in the side seam.

Some leave the shoulder seams open, turn, fold the edges of the fronts' shoulder edges inside (like pants hems). Insert the raw edges of the back shoulders into the fronts and top stitch. Clear as mud?

In the bottom seam, in the back. Then a carefully worked seam can hide the hand work. At least that's the place I used some time ago when I made a vest.

I've done the inside side seam and the shoulders (two different vests), but the shoulders are a little trickier.

i leave the side seam open