I don't think there are any post-child birth vegan women on VBB to give you an answer.
I have had five children thus far. Before my first child I had a 23" waist, but I was also only 18. Now it stays around 26 and is always the last to come in!
Every woman is different. Most women like to carry excess fat in their abs, hips and thighs. One of these three areas (or all in some cases) can be difficult to come in. Think of it like a rubber band that keeps getting stretched and stretched, eventually the elasticity gives out.
The abs should be worked like any other muscle group, and you will have to be very low BF in most cases to get them to show. Depending on your genetics, this can be too low BF to really be healthy.
I would recommend concentrating on BW exercises, several sets (3-4) of 2-3 exercises to failure a minimum of 3x's per week.
Lastly, skin takes time to bounce back as well and that may be some of what you see. If you have quite a bit of stretch marks around the abdomen, most likely your skin stretched beyond what it can come back from and thus you get the proud badge of mother hood.
P.S. - inspiration .... I should mention I used my last child as an excuse to eat and eat and eat! I was 184lbs , my fighting weight is now 119 (very easy to maintain) and I go into comp at 110-112.