It isn't crap, though, is it? HVs are always getting things wrong, but it is correct that if a baby is having formula milk at 12 months, there is no need to give vitamin supplements because the vitamin supplements are added to the formula during the manufacture - check the ingredients list.
There is evidence that children in the UK do not get outside often enough to make Vitamin D, esp if they are black or brown skinned (or even Mediterranean) and esp if they live in the north and esp if they have a modern lifestyle which means they are in the car a lot or inside, at home or at nursery.
There's not much evidence of a lack of vits A & C, but for some reason these vits are packaged with the D in the children's multi-vits... A & C won't do any harm, though.
The mother's own diet will not affect the vitamin content of her breastmilk unless she is severely deficient - but the amount the baby gets will depend on how often he is breastfeeding, obviously. Typically breastfeeding toddlers in the UK do not breastfeed very often.
This is why current guidance is to give vit supps to babies not using formula over 6 mths, and if the baby has moved from formula to ordinary cows milk at a year, then the vit supps should start then.
HVs do get things badly wrong, but in this case, the HV does not deserve the ordure you are piling all over her!