I think it's important to recognise that not all bacteria and viruses are the same. Many are harmless, some are pathogenic, some we get exposed to at an early age and develop natural immunity, others live on a percentage of us (e.g. meningococcus, staphylococcus) causing no harm but if they invade our bodies can be fatal...
Many viruses are usually pretty harmless (e.g. rhinovirus), the viruses that cause mono often cause no illness when people are exposed to them at a young age, and they live inside many people in latent form without causing disease for the person's entire life.
But in some cases when people are exposed e.g. to EBV at an older age, they develop mono and sometimes recover without problems, happily live their lives with the latent infection, while others will develop ME, others MS, and others Lymphoma.
In some countries, exposure at an early age can lead to Lymphoma.
So the interactions between pathogens and hosts are dependent on many factors: the specific species and strain of pathogen, time of exposure, environmental factors, available treatments, immune system factors, epigenetics...
So we can't just lump things like exposure to 'bacteria', 'vaccines', or 'viruses' into one pot.
The factors leading to a person getting mono, and then getting ME as a result, are likely to be multifactorial but also quite specific in my view.
I don't know about the evidence for vaccines reducing the ability of the immune system to fight off e.g. EBV. I haven't looked into this and don't know whether or not there is specific evidence of this so I won't comment.
I personally think genetic factors which affect immune system or cellular function might be more important in conversion from mono to ME than how many vaccines a person has had.
Since most people are vaccinated nowadays, and most people don't get ME, I think it's probably not vaccines, and probably a combination of viral, host genetics, and environmental factors such as age of EBV exposure, stress levels at time of exposure, etc.
Just like why some people get MS or lymphoma after EBV exposure, but not ME. Most of these people will be vaccinated too. And so will most of the people who stay well.
I think we probably have to look in more detail at the intracellular and intercellular virus-host interactions to understand what the causal mechanisms are in post-mono ME, and why these are different from the interactions that happen in people who don't get ME after mono. I.e. I think we have to focus on the biochemical and molecular specifics of this interaction, and work back from there.