Yes, I am aware one can be used for the other. (Although probably not in both directions)
I was wondering if the technological advancements from developing one of these modern marvels paved the way for the other?
i.e. Vibrators are like space tang. Both turned into more widespread commercial successes that, ironically, ended up in the mouths of millions of people. 
← clean teeth
Electric toothbrushes do more than just vibrate; they have an oscillating movable head - i.e. the rotation the motor spindle is translated into a side-to-side motion by a mechanism; as I understand it, vibrators (or at least the simpler ones) just contain a motor with an eccentric weight on the spindle; there’s no oscillation, no elaborate mechanism, just vibration.
Pager/cellphone vibrators work on the eccentric weight principle; they have more in common with sex toys than do electric toothbrushes.
The similarity between the cylindrical shape of electric toothbrushes and that of vibrators is pretty much a coincidence; part of it is because they were both desighed to accommodate batteries of standard sizes, but that’s about as far as it goes; toothbrushes are designed to fit the hand; vibrators are designed to mimic the penis (despite the fact that we’re all aware that insertion isn’t the only, or even the best way to use them)