The device will be rated for someone swapping the wires around. That said, having the wires the correct way around adds a level of safety to the design, and that level of safety is now defeated.
Myself, I replace any power cord that has been damaged. As such, I would replace the device automatically. I have been burned once too often with "fixed" and "okay" cords. People tend to wrap their hand around cords. When you get zapped with current your muscles contract. As such, you can't let go. The problems are worse if the cord may get damp or oily. (I'm thinking of industrial environments here, but it applies outside and inside the home too.) If the cord has developed a fault in one location, then it is likely it will develop a fault in another too.
Additionally, the rule of thumb is that if you are going to get shocked, it is much better to be shocked at only one point of body contact (which is survivable), versus two points which is much more likely to have fatal consequences. Always try to handle electrical equipment with only one hand. Not one hand while leaning against a metal surface, not one hand with feet in water, and never with both hands. Since this isn't the industrial kind of vibrator, two point contact may be possible.
I would probably replace the entire power cord at a minimum, and possibly the whole unit. Actually, unless it was plugged into a GFCI (Ground Fault Interrupted) receptacle, I wouldn't use a line powered vibrator at all. Those GFCI receptacles do save lives, I know someone who almost killed herself, and the GFCI stopped her.