Thanks for the response. I'll try doing some more playtests to get a better feel for it. I'm a chronic over-preparer thanks in part to a lot of "crunchy" RPGs, so I have to work on my improvisation skills and my confidence in them. I think the 3 P's system you mention might be worth looking into. (I'd probably replace "Politics" with "Purpose" in my case.) In my hypothetical score's case, getting the key is the purpose, but finding other profit while they're digging through the vault would be a consideration to balance with getting out before the alarm is raised.
I think one of the difficult things with improvisation is having appropriate consequences to give out, especially on 4-5, but if my FFG Star Wars games are an indication, a lot of players are eager to offer ideas.