In order to make things like embedding Vidyard content in Salesforce Communities easier, a lightning component has been developed and is available on the Salesforce AppExchange, allowing users to quickly and easily add players to a page by Player UUID.
After adding a Vidyard player to a Salesforce Community page and beginning playback of the player, the fullscreen option may not be available.
When adding content to Vidyard pages through applications, components etc. Salesforce typically adds this content into an iframe which may not have the HTML attribute "allowfullscreen". This means that content inside of the iframe will not be able to size the player to fullscreen. In order to work around this restriction, the Vidyard player uses the flash fallback player, which allows the addition of a fullscreen button over the playback area itself. This requires having flash installed on your browser to function but would look something like this:
Make sure that flash is installed on the browser that you are viewing content from. Some browsers, like Google Chrome, have their own flash support built in and no additional installs are required, other browser types may require additional tool installs.