I also have never vaccinated my chicks nor do I use medicated feed, but there's nothing wrong with feeding them on medicated starter if you're nervous about it. Just make sure to follow the label instructions and switch them off of it when it says to!
I have found that it's more effective to expose them to a small amount of soil from around where their coop will be and let them gain immunity naturally, personally. Just take a plug of soil, as dry as possible, and either sprinkle it in their bedding or leave it as a chunk in the brooder and let them peck at it as they please. I do this at ~1-2 weeks of age, once I'm sure they're on their feet and a little more sturdy than right after hatch, and then give them about a week after exposure before adding any more soil to the brooder or putting them out on the ground.
I have found that it's more effective to expose them to a small amount of soil from around where their coop will be and let them gain immunity naturally, personally. Just take a plug of soil, as dry as possible, and either sprinkle it in their bedding or leave it as a chunk in the brooder and let them peck at it as they please. I do this at ~1-2 weeks of age, once I'm sure they're on their feet and a little more sturdy than right after hatch, and then give them about a week after exposure before adding any more soil to the brooder or putting them out on the ground.