What website(s) do you use to find topics to practice Vietnamese? None, of course!
You have things to talk about- everyday things like the weather, TV, current events, work, family, etc. These are exactly the topics you should practice in English. There's no use bringing up a topic so obscure that no one knows anything about it.
Each member of your group should be prepared each time you meet to talk about something- anything. You will probably find some topics take longer to get through than others. That is fine. Maybe not everyone will get to present 'their' topic each session, and maybe two or more will have the same topic. That's okay too.
You shouldn't have to 'control' the group. All members should be more or less evenly matched, or its not an effective group. You may have one or two who tend to dominate the discussion, and you may need to ask direct questions to bring others into the discussion. If someone is not contributing to the discussion, they should not be part of your group.
You may find it helpful to keep some sort of record of who attended and who presented their topic. That way, if someone doesn't begin a discussion this week, you can make sure they get their turn next week.
Student or Learner