Viral load is not the only factor; there is also a) how long that infectious viral load is present for, and b) chance of getting infected with that viral load in the first place.

No analogy is perfect, but I think that this one is instructive, as it breaks the binary logic.

The statement "vaccines don't prevent..." conflates two very different statements, i.e. "vaccines don't prevent all occurrence of $outcome" (which is often true but not the point) and "vaccines don't have any impact on $outcome, prevent any occurrences of $outcome" (which is false, but is often implied).

It's clear that COVID-19-Omicron is infectious to a crazy degree, but that's a different statement to "this defensive measure does nothing".

Only after they get a breakthrough infection, which is still presumably less likely when vaccinated. That still works very nicely with the bulletproof vest analogy.