I want to attach a Vivitek 650HD projector to a ceiling mount, but am not sure if I mount it upside down. There doesn't seem to be a "flip" function to flip the picture.

I am 99% sure most projectors are mounted upside down. There should be a way to simply flip the image electronically via the option menu.

In most projector OCD menu (keep in mind you may want to do this before mounting), under the "Installation" or "Initial Installation" section should be an option about Projection mode/method. The options should allow you to pick front, rear, ceiling, or ceiling+rear. This sets the project to know when it will be in front of the screen, behind it, and/or attached to the ceiling (which it knows will be upside down). Then you simply set it to ceiling, mount the projector, and finish your other adjustments as needed.