Why are Visualforce pages not showing for Salesforce Ideas?
Creating a Visualforce page by following this documentation and then adding the Visualforce page to the Idea object page layout will result in the Visualforce page not being displayed.
Confluence or JIRA version
This is due to a limitation with Salesforce Ideas.
The Salesforce Ideas object page layout is not customizable. While it is possible to view changes in Preview mode, the actual page will not change according to your customizations.
As of this moment, there is no resolution or workaround for this case unless Salesforce allows customization for the Ideas page layout.