Is it a fact that a more flexible arm will create more vibration? I think a flexible arm would swing more, but would actually absorb the vibration. If the objective is to transmit vibration in the water, a stiffer arm would be more suited for that IMHO.
So, to illustrate, if your hold a vibrating sander with a really loose grip, it will move around a lot, but you won't feel the vibration much through your arm. If you hold it really tight, it will move around less, but you will actually feel it a lot more.
Or, an old 70's fiber glass fishing pole sure bounces around a lot, but you don't you really can't read the bottom structure when you drag a worm on the bottom of the lake. On the other hand, a stiffer, lighter graphite rod, doesn't bounce around much, but is excellent at allowing you to feel the gravel, grass, mud, etc on the bottom.
If you agree with this theory, then the best way to increase vibration would probably be to play around with the source or motor of the vibration to modify the volume and frequency. Go with stiff wire, but seek out new blade designs or configuration.