You will need:
1 concrete 5 installation
1 vanilla forum installation
Install your C5 if not done so already.
Install your vanilla forum if not done so already.
Go to your vanilla forum and navigate to: addons / plugins
Select "enable" under the "embed vanilla" plugin.
Go to your C5 website and create a new page called "forums" or something to that effect. Make sure you choose a "full" design with no sidebars.
Grab the iframe plugin from C5 website - http://www.concrete5.org/marketplace/addons/iframe/
Go to your forum page you created in C5 and edit the site and create a block in the "main" area. Choose the "iframe" type (towards the bottom) and when asked for the url...enter the url to your vanilla forum page.
Save your edits to your page.
If you have any questions or comments, please let me know! Enjoy