I think for the vest, you'd be better off reducing the number of stitches on the sides rather than trying to get it that much smaller through needle size for a 3-month old. It has 110 stitches around, with 5 stitches per inch, so about 22 inches.
Check out the measurements of a 3-month vest and subtract stitches according to the measurements of that one, by 5 stitches per inch.
As for the booties, I can't imagine that newborn size is all that much smaller than 3-month size. I wouldn't go up much more than one needle size with the same thickness yarn or they'd probably be too floppy. You could make the ankle bands longer.