With virtually constant action, I don't see how association football (soccer) fans in live attendance can leave the stands for any reason. I am much more familiar with sports that have a natural breaking point in the action. So this is a question that I would like answered directly by those who have solved this by attending a soccer match, faced the need to ingest or excrete, and you had to solve this in the most practical/pragmatic way that kept you from missing potential scoring action.
So let me provide the counter-point. In basketball and American football, there are quarters and stoppages of the clock: timeouts, special teams' actions for transferring possession, penalties and foul shots, and for main events, television/broadcast timeouts. Baseball has over a dozen breaks in the action per game. Tennis viewers and golf viewers have different expectations. But with auto racing, the events can be missed for periodic absences by the obligatory "what did I miss?" question upon returning.
So American audiences are used to finding the restroom during these events, or finding the concession stands.
My observation is that roaming vendors might be able to (partially) solve the latter of the two problems with soccer. But I don't even see much of that going on at soccer matches. The former could be solved (like in American NASCAR) with a BYOB policy. Or perhaps there are enough screens in the concession and bathroom areas now for fans to follow the action.