The best vitamins are those found in a really good, well-balanced diet, unless your vet recommends something specific that your pigeons may need.
There are some excellent pigeon mixes on the market, and the best ones even have different varieties depending on the time of the year and the pigeons' needs -- for example, the best food for the time when they're breeding, or moulting, etc.
This should be supplemented at least once a week by offering them a dish of fresh vegetables: lettuce, shredded carrot, fresh peas, fresh sweetcorn and spinach, for example.
A healthy pigeon will find all the vitamins he needs in a diet like this.
Equally important is to make sure they have the right mineral supplements. In the wild, pigeons get their minerals by picking up grit from the ground, and the sunshine processes that into the calcium, phosphorus etc that they need to have strong bones and muscles, and to raise healthy babies.
Pigeons in captivity need to have a dish with grit and oyster shell, and to be in the sun for a few hours each day (but be careful not to leave them in the sun in very hot weather without a shady place they can go into).
I like using pecking blocks, which look like small bricks made up of different types of grit and with mineral supplements. The pigeons love pecking at them, and it keeps their beaks strong and neat.