Never had the flu jab, never had time off sick. Same risk of carrying and spreading a virus as others. The group is as safe as it is from those who despite having had the vaccine are to host it in their nasal passages, from surface contamination and pass it on in the usual way. Vaccination does not stop the spread as such and therefor individual choice is relevant.
Vaccines stops the breeding of the virus. Most viruses have a finite amount of time they can live outside the host organism. That time ranges from minutes to days. A virus trapped on a doorknob or inside a nasal passage and unable to reproduce is doomed. If that virus is able to travel to another vaccinated person, the odds are again no joy for the virus. et cetera
Vaccines present a portion of the virus or bacteria to the immune system, and when the immune system recognizes that portion as "not-self", it sets off a cascade of actions that result in antibodies being created. In producing the vaccine, the developer of the vaccine has to consider which parts of the virus (or bacteria, for that matter), are essentially unchanging (usually some of virus capsule components and some of the viral enzymes), which are changing quickly (also some parts of the virus capsule), and which are dangerous to have (the viral genome, either in RNA or DNA form). A good vaccine will incorporate a number of components of the virus without incorporating the dangerous elements. The problem for us is that the human immune response is not perfect, and sometimes the immune system will characterize something as "not-self" which is actually "self" - and this get the auto-immune response attacking components of our own body.
As for viruses not being able to live outside the body... I really wish that this would be true. Depending on the viral capsule structure, some viruses can exist in dormant state for centuries. If they are not exposed to UV, ionizing radiation, or oxygen, they can stay dormant but viable for a long time.
An interesting bit of information came out of the genome sequencing study - it appears that more than half of the genetic endowment we carry is made up of "junk" DNA, which apparently doesn't code for anything. Further studies determined that much of this "junk" was very similar to viral gene sequences, leading some researchers to consider that the genome represents millions of years of infections of viruses of the basic eucaryotic cell. There is a link between cancers caused by viruses, and cancers caused by damaged DNA, which seem, in some cases, to re-animate the dormant viral sequences.
Conspiracy theories aside, vaccinations DO reduce the chances of getting sick from a virus or bacteria, but only IF the right strain was used, and if the vaccine developers were clever enough to select those components which are present across a number of strains.