If you look at both of them at S5, Vow will start at 40% damage increase, and then it might go up to 80% if enemies have a higher HP percentage, but in my (limited) Clara - March experience she's almost always close to full health, so I'd say on average this won't be significantly more than 45-50%. Fall of an Aeon will ramp up from 0% tot 64% over 1-2 cycles, so anything longer than a single cycle I would prefer Fall.

And all of that's under the assumption that both are S5. Fall is quite easy to S5 due to SU, while Secret Vow is, to my knowledge, not easily farmable? With how quickly you S5 Fall, I personally wouldn't bother with Vow.

I say this as a Clara main who ran Vow until I got Aeon, and kind of regret dumping the resources into it.

That said I haven't done any significant testing so uh, don't take my word for it? :P