The 8125 (Cingular's 6700) has a great feature where one of the buttons is can switch the phone to vibrate and back. There is a button assigned to the wireless manager. If you press it quickly, it will start the wireless mgr which is what you can do in xv6700, but if you hold it, it puts the phone into vibrate and back. There has to be a way to do this in 6700.

There are programs out there that let you create Audio profiles, such as Loud, Normal, Quiet, Vibrate, Silent, etc.
Maybe some of these type of apps let you assign a key...

How would this be much different then using the slider to adjust the volume down to the Vibrate option?

Toggling Vibration Mode with a button press of your choice can be done easily with AEBPlus

It's not much different, there are other quick ways but a dedicated button is always good. You can switch it without taking the phone out of your pocket. Also, that switch is kind of annoying.