You can go to Setup > Create > Tabs and create a new VisualForce tab with the VF page that you want to use as the home page.

Then go to your Customer Portal > Customize Portal Tabs > and add your new VisualForce tab to the Selected tabs and set that tab as the Default Landing Tab.

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Salesforce MVP | Lightning Champion | 16x Salesforce Certified | Application Architect | 6x Trailhead Ranger | Techforce Services | Australia