Some strains of the flu do more damage to the young and healthy than they do to babies and the elderly. Hey, if all these anti-vaxxers want to risk death, that's OK with me. But, the sad part is that rather than get a flu shot, they will walk around infecting hundreds of innocent bystanders until they finally themselves end up in the emergency room.
Fact: if you ingest the flu virus into your system, you will get sick from the flu. Vitamin D does not kill flu virus. No amount of vitamin D or anything else will will keep you from getting ill. You can fight the exposure risk by washing your hands a lot, and staying away from other people who are contagious. But, that's hard to do since infected people don't walk around with a sign around their neck. And, by reading the comments here, you can easily tell that there will be many infected people walking around with the flu this year as every year, and infecting other people.