The core issues I had with it are that the scenario he's envisioning just isn't playing out in the real world.  He has a mostly coherent model he's working with, and he's making the valid claim that "for some set of parameters and constants, terrible things happen!"  It's just that in real life, the parameters and constants are nowhere near what's needed for the badness he's claiming.

Specifically:

• His scenario requires a much higher mutation rate than we're seeing;
• His scenario assumes vaccine immunity isn't very strong, but what we're seeing is incredibly strong immune responses;
• His scenario assumes that natural immunity (from being infected) is significantly stronger than the vaccine generated immunity, which doesn't appear to be the case.

Basically, none of his assumptions are valid, and so his final prediction is for a reality that isn't ours.

I found this video interesting and quite concerning. 

Vanden Bossche makes two arguments: 

(1) The ongoing mass-vaccination campaigns are poorly timed. We started vaccinating right about when lots of concerning new variants were showing up independently in different locations, suggesting that SARS-Cov-2 is quick at evolving. The vaccines are targeting outdated variants, and some vaccines are already only partly efficient. This creates the perfect conditions for further viral evolution. Therefore, we should expect immune escape really soon. Booster shots may help temporarily, but that's not a good solution because you're always a step behind, and if the outbreak isn't under control at any point, you just keep pressuring the virus to evolve and you thereby make it better at evading antibodies.

(2) There are two types of antibodies: 'innate immunity,' which is based on undiscriminating antibodies, and acquired immunity, which you get from the vaccines (or from having had the virus previously). Innate immunity is why young people do very well against the virus. Now, when you give people specific antibodies from the vaccines, those antibodies will still bind to the virus, but they won't neutralize it. They will be useless, but they'll crowd out the less discriminating antibodies from innate immunity, the ones that would actually work against the virus. This way, vaccinations could end up harmful. 

My impression is that his points in (1) seem undoubtedly accurate and pretty scary, but I think it's plausible that update vaccine shots will be made and distributed quickly enough to at least keep things under control (similar to Influenza each year). Besides, I don't see a good alternative. (Given that trying to eradicate the virus globally requires an unrealistic degree of willingness and coordination abilities.) 

I lack the expertise to judge his arguments in point (2), but there's something at 1:08:20 in the video that Vanden Bossche says that makes me think his mind is ideologically clouded. He talks about 'natural immunity' in this hyped way and suggests that 80%-85% of people "don't get any symptoms." I think that's just false. Asymptomatic infection is <50% with Covid. So given that his entire argument rests on understanding innate immunity, and given that he gets a central fact about it wrong in a way that suits his biases, makes me think he may not be right about these concerns. 

Of course, people can be wrong about some details and still be right about the general picture. I do think the mechanism he proposes sounds at least plausible to my lay ears. In particular, I think the situation "mass vaccination campaign during a global pandemic against a fast-mutating virus" is quite unprecedented, so it's not crazy to think that policy makers may not be thinking about virus evolution and immunity mechanisms in fine enough detail to realize that they're creating a dangerous mix of circumstances. 

One thing I'm skeptical about: If his concern with working antibodies being crowded out were correct, wouldn't we see instances where anti-flu vaccines end up harming people, because they'd also crowd out antibodies from innate immunity? But this is basically never the case, no? If you gave an outdated flu vaccine to a young person, they wouldn't do worse against the current flu virus, would they? That's another reason why I I'm skeptical, but I don't understanding anything about the specifics of the immune system.

Similarly, what about previous infection? If someone got infected by the original Covid variant in 2020 and then reinfected with some future evolved Covid variant that's very good at evading previous antibodies, it seems like Vanden Bossche's model would predict that they're going to do worse than if they had never had a previous Covid variant. Would we actually see that in reality? So far, antibodies seem to always be good to have.