So what you wanna do is make sure your fallout shelter is modded with infinite resources (this is how i did it, or you can waste your money on nuka-cola in the shop and get a ♥♥♥♥ ton of it).
Now follow these steps below:

1. Skip the middle or bottom tier daily objective until you get an objective rewarding a lunchbox.
2. Complete that objective.
3. After completing step 2, quickly go back into your daily objectives and skip the new objective you're given 2 or 3 times and you should be able to get another objective rewarding a lunchbox consistently. (note, the top daily objective does this too but has a lower chance of giving a lunchbox reward than the middle and bottom one)
4. Repeat steps 2 - 3 until you're satisfied with your amount of newly gained lunchboxes.

I want to point out that 8/10 lunchboxes obtained through this method will 96% of the time contain at least 1 legendary Junk, Weapon or Outfit - vault dwellers do get dropped but are often rare in quality.