You can, but I guess it doesn't really deal with the problem fully! All that water could be sitting on the metal just inside the surrounding area of the vent, so in the long run you might get a rust problem. The water doesn't really come in from the bumper, it runs down the back of the tailgate next to the lights I believe.
When I did this fix I didn't take the bumper off (I dare not touch it the way things are going right now), I sealed around the vents from inside. It's not the correct way to do it, but right now I'm just happy that the boot is dry (shame about the rest of the car)