You haven't looked in the right place on GenVibe:
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The image you included illustrates the factory-option 17" wheels for 2003-2008 models, not 18"
There were no factory 18" wheels offered for 2003-2008 Pontiac Vibe models.
The 18's offered for the 2.4L 2009-2010 Vibe models are 5x114mm bolt circle, and don't fit the 5x100mm bolt circle of the 2003-2008 models.
Factory steel wheels with hubcaps were offered in 16" size only for 2003-2008.
Factory alloy wheels were offered in 16" and 17" sizes for 2003-2008.
Wheel and tire vendors such as TireRack.com have a huge database of compatible wheels and tires available in sizes to fit your Vibe, including 18". Just select manufacturer, year, and model before browsing. They also have car and wheel images so you can 'see' a simulation of how the wheels would look on your color and model of car.
EXAMPLE: