Amount of blood vessels in area aka vascularity decides where would we administrator any parenteral product which includes vaccine. We want to it to get dissolved in blood to show it's effect, if there are a lot of blood vessels the distribution of vaccine/medicine will be faster. But vaccine isn't injected into blood directly since it's a killed antigen Or it's byproduct or any other live antigen, we want it to act slowly so given intramuscular, because if it went directly in blood the action will be faster that's the side effects I'm talking here like fever, etc who are going to cause problems. I've not heard or seen anyone administering vac in foot area though.