Obviously, destroying vaccine-vulnerable strains of a virus will leave the vaccine-resistant ones to represent an increased fraction of the overall viral population. I'm asking, though, whether/how vaccination can allow a particular strain of a given virus to reproduce more than it would do without vaccination against another strain of the same virus, or to infect more hosts than it would infect without such vaccination.
If only vaccine-vulnerable strain A exists, might application of an anti-A vaccine somehow directly cause the mutation that creates resistant strain B? If so, how? Or are such mutations only random (in which case strain B might randomly arise even if the vaccine it was resistant to was never even invented)?
If strain A and strain B both already exist before any vaccine is developed, and then strain A is perfectly eradicated by a vaccine that strain B happens to perfectly resist, then of course strain B will remain - but will it actually benefit, in the sense of becoming able to increase its own population or its host population beyond what the respective count would have been without the anti-A vaccine? If so, by what mechanism?