OK first of all, it is rather unlikely that you will be checked for the yellow fever vaccination at a major entry point. What the rules say and what happen in practice are often very different.

That said if the fact is picked up that you have not been vaccinated, it is more likely that they will force you to recieve the vaccination at the airport rather than deny you entry and ship you straight back home.

The other point to consider is that the restriction is there for a reason, evading the legal issue is one thing, but it is surely very prudent to get yourself vaccinated if at all possible. Yellow Fever is all over the place and so it is definitly a top 5 disease to get a vaccination against if you are travelling anywhere it remotely exists.