The virus mutates to survive. Meaning...when it can no longer make people sick, it drops back and re-arms. Immunity, whether from the vaccine or natural antibodies, is what causes mutations.
You are giving the virus too much credit for intelligence. Vaccine Immunity just speeds up natural selection of the mutations that occur naturally.
A virus can mutate any time it's reproduced. It reproduces in anyone who has the illness, vaccinated or not. The more reproductions, the more likely a mutation if the RNA/DNA strands are weak.
The RNA/DNA in the virus strand can be weak or stronger depending on the virus. Smallpox, measles, and other major virus are strong and haven't changed in many years. Thus, the same vaccine you got years ago can still protect you and we've pretty much wiped out the virus here. Covid (it appears), Flu, common cold, mutate often, and the antibodies formed against one mutation may or may not activate against another mutation.
That mutation can make the virus less likely / unable to infect people and the mutation dies out. The mutation can make the virus last longer in air before dying, thus make it easier to catch. The mutation can make the virus attack different parts of the body and make it more deadly, even if it doesn't change it's ability to infect people. The mutation can make the virus look different enough to the antibodies (vaccine or naturally created) that you can be infected again. If it does the last, then we have another outbreak, immunity or not and it will become the primary strain because the other versions can't reproduce as much. We will never know how many mutations have occurred that failed.
If we have a vaccine that protects against the Alpha and Beta strands, those find it harder and harder to find a host to reproduce in and die out.
If we have a vaccine that only partially protects against Delta, then it can reproduce more than Alpha and Beta, even in the vaccinated, and become the primary strain. If we reach herd immunity before another mutation can occur, then it's great, because without lots of cases, the chance of a mutation goes down greatly. But if a mutation occurs that allows the new strain to get around the antibodies of the vaccine or natural antibodies, the circle starts again and we have another wave of cases.
The virus isn't reacting to any pressure, natural selection is allowing the mutation with the best attack to succeed.
Now, and you will want to get your tin foil hat out for this part I'll admit, I'll note that for years many new Flu seems to mostly come from China to the world. Was this an accident or has China been testing viral attack for years? Now covid show up. a form of flu -- how hard would it have been for them to breed various mutations of the Covid virus and every few months fly a new version to some part of the world and release it so they don't get pegged as the source? There is no way for us to tell if the mutation was in the wild or in a lab somewhere. Then they claim the virus is causing their chip manufacturing to fail, so they can't sell us items our industry needs thus wreaking our economy and society more? You don't hear people complaining about the trade deficit or what China is doing in Hong Kong now do you?
This whole thing seems to be giving China everything they ever wanted too much for me not to have doubts.