I'm pretty sure I've never been attacked by raiders until I have a dozen or so vault dwellers, although I haven't tested this properly yet (I normally grow my vaults quite quickly). I'm aware Deathclaws are 'unlocked' at a certain population (60 for regular and 35 for survival vaults) - is there a similar, lower threshold for raider attacks, or are they timed, or does it depend on vault caps, or a certain room, or something else?
Or have I just been lucky? I suppose it's possible, but I assume there's something preventing attacks in e.g. the first 60 seconds of play...