...and was rendered unusable (it says so on a terminal inside the vault).

So

1) How did the dead scientist get where she is - straight outside the vault cave entrance? The vault entrance is collapsed and unusable, after all... Surely she didn't trek all the way through Little Lamplight? And then what would the point be to go to the cave entrance, which they already knew were nuked and unusable?

2) Who put up the "high radiation" warning signs we see close to the vault? Surely not the super mutants, since they are immune to radiation...

Seems she was a curious wasteland scientist (definitely not a Vault Dweller) that made it through the radiation field. But then collapsed upon reaching the door.

Now as to why she did this, that's up in the air.

My best guess, and the only guess IMO that makes any real sense, is National Guardsmen. We see in Fallout 3 that National recovery efforts were underway in the few days after the war. Before everything began to fall apart. The National Guard were setting up checkpoints everywhere and of course Germantown was being used as a primary aid center staffed with aid works and patrolled by NG forces. More guard units were also being trucked in from Pennsylvania, having driven "all night" to reach the D.C. area.

Pinpointing areas of danger and concern would have probably been one of their established directives. And similar signs can be seen set up in other National Guard checkpoint areas IIRC which supports that notion.

But of course recovery efforts quickly fell apart, since the Guard began abandoning their posts because of the chaos along with dwindling supplies and the onset of radiation sickness and the spread of fallout.

Not a perfect theory, but its something.