I need to install an octagon gable vent in gable end of my garage. It has vinyl siding.
Do I remove the vinyl siding and then install or do I lay it out on the siding and cut through siding? There is the center support of the truss that I cannot cut out. Just set the circular saw to the depth of the osb sheeting? I have searched the internet to find a good site with illistrations. No luck.
The garage already has ventilated sofit and ridge vent but the shingles are starting to curl.
No insulation is blocking the ventilated sofit on the interior. Already checked.
Any assistance would be appreciated!

You will likely chip some of the siding when trying a plunge cut so removal would be the cleanest job. Does you building already have soffit vents in the eaves? Most roofers will tell you to not install a gable vent if you already have a ridge vent, they can pull in snow or rain.

To cut any siding easily use a plywood blade and put it in the saw back wards. This makes real nice smooth cuts. The turn the blade around and cut the Ply/OSB. Now you can sawzall the rest or cut from inside. Leave a little room for the (J) channel to install before you set the vent.

Put plenty of them in. Knife the corners if you are not sure of the steadiness of the cut.

Thanks guys... great advice..
What is the best way to center the octagon vent in the gable?

Just treat it like a square window. Width and height as it were a regular window. Cross center from those corners. Even though it has 8 sides the width and height are the same.

Again just measure the height and with light pencil square on siding now cross corner that square to find center.

If you plumb down from peak and use a level on that line there is the 'Square part to start.