Hmmm… I see why you are trying the iframe / inline combination. To make it work, do this:
1. Use class=”fancybox” instead of fancybox-vimeo because you are trying fancybox in inline mode (with embedded iframe, yes, but still) as opposed to iframe mode which is used for vimeo.
2. Remove the lines
<script>!window.$ && document.write('<script src="https://www.dreiwuenschefilm.de/wp-content/themes/iA3 1.2.1/assets/js/external/jquery-1.5.1.min.js"><\/script>');</script>
<script src="https://www.dreiwuenschefilm.de/wp-content/themes/iA3 1.2.1/assets/js/external/jquery.fancybox-1.3.4.min.js"></script>
from your themes footer.php
Then the test link should work. But be aware that clicking the same link a second time will show an empty box… This is a side-effect of using iframe embedded in inline content. There is a way around this ‘issue’ but it would involve hacking the plugin to create two new onComplete and onClose event function parameters as suggested bu JFK on https://groups.google.com/group/fancybox/browse_thread/thread/d7321b9a4df5b20b/