Wil you asked abut Vit K, right?
For me, the risk isn't big enough to give something we know little about to a baby. The chances of late-onset VKDB are roughly 4 in 100000. Babies are given intramuscular vitamin K because it is easiest and cheapest- not necessarily because it is 'best'. There are other oral preparations available, but these are less widely used, and won't stop early onset VKDB. Additionally, the shot contains preservatives and bovine derivatives (as you might expect), so if you are veggie there is no alternative option for you ('fresh' vitK is only available in the US), and if you are determined to BF because of the associated health benefits it seems counterintuitive to happy introduce something unnatural into your baby's system.
A lot of historical studies were very badly carried out, with small sample sizes and lots of flaws. Naturally they threw up all kinds of results, including a higher incidence of ASD in those babies who had received the shot. This has since been disproven but it does show that we need a study on the contemporary preparation.
Babies are born with subnormal levels of vitamin K, but that is when compared to adults. No research has yet been completed about why all babies might be born with this level of vitamin K, rather than the normal adult level, and I just think that there must be a reason; probably something to do with clot prevention when in the birth canal, or placental transfusion.
Again, vitK doesn't cross the placenta, and there must be a reason for this. It is also relatively low in breastmilk, even if the mother is taking a vitK supplement, and I think there is some physiological reason. I just want to see some proper research first before I make a decision.
Finally, I disagree with the lack of information readily available to parents antenatally (essentially fuck all) and disagree with the conversation when offered vitK; it's usually along the lines of:
MW: would you like the vitK shot for baby?
Parents: Err, what's that
MW: well, it's a vitamin that stops baby bleeding to death, would you like it?
Of course, all this goes out the window if you FF, because formula is vitK-enriched.
However, this is all research-based, and I may completely change my mind when the baby gets here!