I lost my vault (boosted by buyed lunchboxes) before half a year due to mobile phone crashes. Answer from Bethesda support was "sorry, game has offline save".
I decided to install Android emulator AndyWin into my PC and play Fallout shelter. I ends with creating backups when they update game with cloud-save. Wow, finally something, finally online save which can be restored.
Now, after half a year, when i boost 100+ dwellers' SPECIAL into "all 10", with lots of legendary weapons and outfits, with lots of coins and fully builded vauilt, my AndyWin crashed during upgrading and i had to use that 6-month old backup for restore. And Fallout shelter save file? Sure, loaded the old poor vault from 6-month past.
So my 6-month progress is away. And answer from Bethesda? "Try update game, try reboot device and check if you not altered the system clock. If this not help, sorry, you lost your vault."
So stop thinking that cloud-save can save your vault. If you have bad luck, you will lost all.
This game is great, but not for me. No more starts from begining.