If quarantines come to pass, it probably won't matter much whether you've been vaccinated or not — Nobody's going to have time to look over you medical records, much less confirm that they haven't been altered or forged. If you're unlucky enough to live in an area that's slated for quarantine, that's most likely what you'll get.
Small pox spreads very rapidly and has a short incubation period. Yes there may be some regional quantine if the outbreak is limited to that region but I am talking about those that refuse to get the vaccination and contract the disease. There will be many "rugged individualists" that will insist there is no authority for the government to completely isolate them. Rather than argue with them, they will likely just be shot on the spot. Small Pox is one of the most hideous disease known to man and quarantine is not an option that can be debated.
Because of this and many other civil defense reasons, I'd suggest everyone have a couple week's worth of food and water on hand in their house. This isn't as difficult as it sounds. I've got a 55 gallon blue polypropylene drum full of water (with an appropriate amount of bleach) lurking in my crawl space. IIRC, it was less than $100, including the little siphon pump.
Bioterrorism quarantine, natural disasters, threats to the water supply - there are lots of good reasons for this sort of home front preparedness.