It is not required for videos to be horizontal
While it will look better if all the videos are recorded horizontally, it is not necessarily an issue if a Tribute participant submits a vertical video.
If you don’t want any vertical videos, you must email the invitees who sent in the vertical video and ask them to re-record their video horizontally.
You can easily send out an email contacting the invitees who submitted vertical videos via the “Send Reminder” tab on the Manage Tribute page.