My fan has a light fixture (globe) under the fan blade motor. One of the screws holding the blade bracket to the motor has loosened causing blade to vibrate. Does the entire light fixture have to be removed in order to tighten screw?

Typically yes.

In many cases you can remove the globe and reach the screws, normally 3 of them. I'd retighten all of the blade screws. If you can't reach the screws with a straight driver try getting on of these Right Angle Screwdriver and Bit Set 8pcs (http://www.cmlsupply.com/right-angle-screwdriver-and-bit-set-8pcs.html)