I think the news stories about the Oxford vaccine have been widely misinterpreted, including by the news media itself. The problem isn't with the vaccine, and the virus isn't disappearing, it's that the vaccine trials are NOT challenge trials. In challenge trials, the vaccine recipients are deliberately exposed to (or in some cases injected with) the virus, so that each vaccine recipient is exposed to the disease. However, in many cases, including that of the covid-19 virus, challenge trials are said by many medical authorities to be unethical. (A doctor from Columbia University in New York, whose name I don't recall, but who comments for CNN, has made a strong case for the unethical status of challenge trials.)
The problem for the Oxford vaccine is that a relatively small proportion of the British public actually has the disease. As the social distancing and masking efforts have cut the rate of infections, it's simply difficult for those people who have been given the vaccine to get exposed to the disease. If they can't get exposed, the vaccine can't be tested.
As excited as people get about the spread of the disease, it's important to remember that right now, only about 1/2 of one percent of Americans are "confirmed" to have, or to have had, the disease. Even if the disease is much more widespread, the actual rate is probably still low. At 10x the confirmed rate, that'd still be only about 5% of the population. I don't know the rate of infection in the UK, but I suspect it's probably in the same range as that in the US.