Best case scenario of the vaccine is that it gives you lifelong immunity.
a) No, it can still kill you
b) No, it doesn't. It's around 85% effective, so 15% of people will still contract CP despite having had the vaccine AND immunity wanes so you don't know when you are no longer immune and need a booster meaning that you may be left vulnerable when you are an adult and there is a greater risk of complications. Also, the vaccine is less effective in adults so your boosters are less likely to work when you need them most.
Re-infection with CP after natural infection ranges from around 4-10% iirc so still more effective than the vaccine.
"Your child is less likely to contract a disease if they have a healthy immune system, but that doesn't mean they are at less risk of complications."
Link to support this please? Why wouldn't you be less likely to have complications if you are healthy? Children with suppressed immune systems are more likely to suffer complications.