You should be fine on the booties with the slightly larger needle and making them about a half inch longer - here's a baby foot size chart
For the vest, you could probably use the same yarn and needle size and the sts for the original pattern, but I think you could make a 6 mo size; most babies can wear that at about 3 mo old, which is when they've outgrown the 3m size. Here's a chest size chart
, you want it at least 2" bigger, 4" is better. If you want it in heavier yarn and larger needles, take out enough sts to get the chest measurement you need. The cable st pattern is 8 sts plus 3 between them, so you'd probably cut down by 11 or 22 sts to make it smaller.