One display, clearly labeled with a different color, different font, using degree symbols vs. "ft/s" units - you're saying this is bad, but two displays right next to each other, labeled similarly, isn't?
[ ] Mode [10.0000 DEG]
[X] Mode [32.1234 FTS]
[10.0 DEG] [32.1234 FTS]
Sometimes you can use modes to hide unnecessary information, but for something that answers the question how fast am I falling you really don't want ambiguity or cognitive load.
What people have to understand is that it doesn't matter how clearly you think you're labeling it, if it breaks the user expectation, it's not clear at all - and an experienced user reads even less, which is the case of a pilot.
Once the pilot develops an habit of, e.g., switching the vertical speed display from feet/s to degrees during final approach (and that may have been what happened), and one day he forgets to switch, or the mode button malfunctions, or double clicks by mistake... there you have a recipe for disaster.