I'm all for modding up stuff just cuz you can so more power to you wanna try.
I think it's going to be much harder than you think it is though. Firstly the hardware is missing so best case you find something that will physically work, either internally or externally and actually get it wired up. But then you have the problem that this device software isn't built to vibrate. Hopefully all the bits are there and you can just find some flag to enable it but more likely the kernel won't support it because it was never coded to handle it. I'm a developer, not Android, and I know when I build something I don't include code that I don't need. Infact I will remove code I don't need if I'm pulling it from elsewhere. No sense in having stuff in there cluttering up and confusing things when you have no use for it.
So you are going to probably be mashing up kernel source from another device to try to get it working. Expect to soft brick (hopefully not hard brick, although that's not easy to do) the device a few times during that process.
The bigger deterrent for me would be that if you do manage to get this working, you're now on you own to maintain it all from a software perspective. Meaning when a new version of Android is released and you want to upgrade you are on your own to merge in your own changes that enable vibration into the kernel and OS before you can upgrade.
Good luck, and post back if you do manage to get it wired up but I think it's going to be a big project. Personally, I hate all notifications on my tablets, including my Transformer Prime which has vibration. I just let my phone handle all that for me so I'm not dismissing them in 3 different locations.