If you're trying to complete the VoD viewer mission, but finding it not progressing, try the following troubleshooting steps before writing in:
1. Disable any adblockers you may be using, and restart your browser
2. Re-login to the LoLEsports website
3. Ensure that you're watching at least 10 minutes of the VoD; if the VoD is shorter than 10 minutes in duration, try to find other VoDs that are 10 minutes and longer
If it's still not working, you can contact Riot Games Player Support asking which VoD's may have already been watched and which have not. And finally, you can also try to use the League Friends app to get your VoD credit.