Chances are your vents already have mosquito netting. Adding wire mesh to the outside of the vents will prevent squirrels and raccoons from breaking inside. Anchor the screen with screws and washers. Painting the screen will make it less noticeable. Keep your home in good repair.
Animals are opportunistic.
If they see a hole they will use it. Repair damaged wood and broken soffets. Don't make it easy for a squirrel to chew into your home. Don't allow branches overhang your roof. Animals will always use a tree limb or climb a tall bush to access your home before they climb the building.