Neither of these things are guaranteed to happen if you get the flu vaccine when you are sick, but they are possibilities. If you have just a mild illness, there is no reason to wait to get vaccinated.
Early in the day, let roast stand at room temperature at least 1 hour before cooking. Preheat oven to 375 degrees. Rub meat ... the outside and rare in the middle. Meat will be very brown ... stand at room temperature for at least one hour.
In the oven, set the temperature to 250 F and let your roast cook undisturbed for six to eight hours, depending on its size, until it's fork-tender. If you want to crisp the skin -- a guilty pleasure -- remove the roast, raise the oven to 500 F, and return the roast to the oven for 15 minutes at the higher temperature.