I thought the RDA was 400iu. Anyway, the RDA for vitamin D is almost certainly wrong. It's interesting looking into how these numbers are determined.
If you are sensible then you want to get a blood test from your doctor and not just take the pills, but I'm more than happy to experiment on myself.
When I embarked on the experiments I was leading a lifestyle in a latitude that made vitamin D sufficiency an impossibility (same for many people). So it didn't feel like a huge risk.
Steve Gibson is not a doctor but a computer scientist but he can explain things such as Vitamin D: