I was told that if you’re breastfeeding then prenatal vitamins are best to take. But I don’t know much about actual post natal vitamins.
If you think your iron levels might be low, it's worth going to a doctor and asking for a blood test as iron supplements can cause constipation, etc so you only want to take them if you definitely need them. The tiredness might be hormonal instead, for example. The vitamins you need will really be personal to you as it will depend on what your diet is like and what's lacking.
I have been taking pink stork postnatals since about 6 months pp. Before that I was still using my prenatals. I also take other supplements like D3 with K2, a beef liver supplement for iron and a minerals supplement. I am still BFing my almost 18 month old.