I'm not one to use tactics like this over every disagreement or argument. But when you have made your position on vaccines clear and they keep bringing it up that crosses the line. It is no longer an argument, it is an attack. If she is going to constantly abrade you until you break you need to fight back, and not with logic because you are way past logic at that point. Her goal isn't to reason with you but to force you to do what she wants with constant unrelenting pressure. Make her regret it. Make her expect it before she says anything, that is the only way it becomes a deterrent. That means you have to be consistent.