If you need a custom button to invoke a Visualforce page, set the button's Content Source to URL then configure the URL as follows:
/apex/[page name]?id={![object API name].id}
where [page name] is the name given for the page, not its label.
For example, to create a URL button on contracts to invoke the contract Create Renewal page the URL would be:
/apex/ffbc__contractcreaterenewal?id={!ffbc__Contract__c.Id}