Well, that didn’t exactly work. Per your instructions, I moved both files up one level to the vantage folder. When I tested it, the main portfolio page was blank again. I then put a duplicate of the portfolio.php back into the templates folder, and now it looks like both the main page and the detail pages are displaying properly. Not sure why, but looks like the portfolio-post.php needs to be in the vantage (theme) folder and the portfolio.php file needs to be in the templates folder (or both) for this to work.
I would also love to be able to display the featured image thumbnail associated with the event directly on the fullcalendar. Are there any plans to add this capability?