The most common knitting technique to make a baby vest is to top down knitting technique. You can easily knit the baby vest in one piece, because you need to cast on less stitches when compared to big ones. Also since you do not need to make any needlework to attach the sides, the vest will not harm the sensitive skins of the babies.
While knitting with top-down technique, the most important detail to pay attention is to divide/seperate the number of stitches of arms and body parts of the vest carefully. If you wish, you can continue to knit the arms and make a cardigan, or continue from the collar and add a hood to the vest or jacket. You can modify it with respect to your need or how you like it.
TOP-DOWN KNITTING BABY VEST SIZES AND NUMBERS CHART
Baby Age Colllar Stitch Numbers
0-3 months >>68 stitches—-10,12,20,12,10__ +4 (increasing)
4-8 months >>84 stitches—-13,14,26,14,13__ +4 (increasing)
8-12 months>>94 stitches—17,11,34,11,17__ +4 (increasing)
1-2 years>>104 stitches——19,12,38,12,19__ +4 (increasing)
3-4 years>>128 stitches——23,16,46,16,23__ +4 (increasing)
5-6 years>>140 stitches——26,18,52,18,26__ +4 (increasing)
Collar to underarm measurements;
3 months >> 12 cm.
6 months >> 16-18 cm.
12 months >> 21-22 cm.
18 months >> 23 cm.
24 months >> 25 cm.
4 years >> 26 cm.
5 years >> 27 cm.
6 years >> 28 cm.
(These measurements are valid for the yarns that can be knitted with knitting neddles Nr. 3,5. You may need to change the number of stitches with respect to the kind of yarn you will use.)