I know what you're driving at and it's all pretty interesting - but counterintuitively I think it's way more effective overall to reduce transmission than it is to reduce severity.
If you make the disease half as deadly, then pretty simply half as many people die. But if you make the disease half as transmissible, then the compound impact means that far fewer than half as many people die.
I think you undercut your own point for no reason.
Less deadly diseases get less reverence, leading to more risk taking. The percentage of people who die goes down, but the number of cases goes way up, resulting in potentially greater loss of life overall. Look at how cavalier we are about influenza, and then we set policy based on whether things are worse than the flu.
This has happened with other diseases.
On the other hand, if the vaccine reduces the transmissibility of the virus, then everybody collectively benefits from each additional vaccination.
While it seems rare, it also seems like a much deadlier outcome.
One of the reason there's a lot of gas behind rolling this vaccine out quickly is that we want to avoid allowing the virus enough time to mutate up some different strains that this vaccine isn't effective in preparing us for since that makes the vaccine far less effective overall.