Even if a vaccine is available next year for COVID-19, it could take years to vaccinate the world.

That is a big “IF”. There are many issues with finding a vaccine for coronavirus. In very short some include:

There has never been a vaccine for a coronavirus (one of the main causes of the common cold),
It usually takes many years to develop a vaccine,
For a vaccine to be effective, a large portion of people need to “seroconvert”, that is develop immunity to the vaccine. Recent studies have show that only 60% of people who have acquired COVID-19 through the community develop protective antibodies, and
The vaccine needs to produce long term immunity. Some studies have shown that in those who develop antibodies to COVID-19 after infection, that immunity wanes by 23-fold within only a few months. This may require multiple booster vaccinations at 2-3 month intervals.

With the above concerns aside, there are other issues too, like:

There are about 8 billion people on earth. At best, if all world vaccination production is switched to producing a viable vaccine, the world vaccination manufacturing capacity is estimated at 6.4 billion p.a. If, those vaccines require 2 or 3 doses, then that is a few years of vaccine production at maximum capacity to vaccinate the whole world.
Then there is the challenge with vaccine distribution. Vaccines need to be kept strictly between 2 and 8 degrees C. It is estimated that 50% of world vaccines fail the “cold chain” and so if there is 50% wastage, then world production would need to further double. How do we get the vaccine between 2-8deg C to remote areas of Asia, Africa and South America, just to mention some remote locations?
Will vaccine distribution be equitable worldwide? If wealthy countries buy up the supplies of the vaccine, and poorer countries which are in hot spots cannot get supply and not for a long time, then the virus may continue to rage on around the world for longer too.

Getting a good vaccine, with minimal side effects, that produces long-lasting, high levels of seroconversion to the whole world will be herculean effort.