No vaccine is 100% effective and Thailand isn't willing to accept any risks. They won't allow anyone in without quarantine until the country they are coming from has been covid free for a while.
just judging by your name, i'm guessing you aren't from one of the handful of asian countries (with near-zero covid) that qualify for the golf asq. but if i'm wrong, the most important thing is to choose a good caddie because you have to use the same one for the whole 2 weeks!
If you book a normal room without quarantine services, you won't get a certificate of entry, but you can book an ASQ package on Agoda, which is what you need.
so far, it seems none of the variants are different enough to limit the efficacy of the vaccines but it's definitely a risk that a more serious mutation could happen before the vaccine is rolled out globally, especially as most Western countries still seem happy to allow the virus to transmit quite freely among their people and across borders.
one person being vaccinated doesn't mean much. to get the virus under control in Western countries, nearly everyone has to be vaccinated. and before countries in the East (which mostly already have the virus under control) reopen their borders to Westerners they will vaccinate their own populations.
no, of course not, the vaccine doesn't even work for 1 out of every 10 people, and the other 9 can still be spreaders, and Thailand remains committed to eradicating the virus, again.
mask wearing and real quarantine of the infected or potentially infected (including new arrivals at the border) are the real keys to keeping the virus death numbers down. social distancing is just an added extra protection (although it is adopted as an inferior substitute in western countries where for some reason they don't want to follow the basics that worked so well in the far east). it wasn't my intention to judge you for choosing to travel during this time (i'm not a saint) and sorry if it came across that way, just a reminder to be aware of the potential consequences.