A lot of people use online tools like JSFiddle or CodePen to test and share Vue problems.
It would be a real aid to debugging to enable Vue Devtools to look at frames other than the main window, or even have it to this automatically, per site.
I see there is also issue #115
I see there is also issue #115
Which means you just posted a duplicate.
Also, the problem has been identified. vue-devtools does not support iframes.
See this issue on jsfiddle's tracker if you wish.
Closing as duplicate