Well, as you're probably already aware, there's quite a number of things you can talk about in the Vietnam War. Your preliminary research should help you narrow down the things that you're interested in. For instance, do you want to focus on the war itself, or the impact of the US public? If you wanted to look at the war itself, there are several controversial issues, such as the use of Chemical warfare, the Vietnamese people's perceptions of the US soldiers/Viet Cong, and so on. For those topics, you could probably pick an issue and see how it impacted a certain event. For instance, "how did the use of Chemical warfare impact the perceptions of the war on the part of US citizens?" or "How did popular sympathy for the Viet Cong detract from the US' ability to establish a stable regime in the Vietnam War?". On the "home front" side, you can look at things such as "how did media coverage alter US citizens' perceptions of the war" and "How did the national teach-ins/student protests impact the end of the war?", or even "to what extent did media coverage enable the mass student protests against the war?". As you can see, there are quite a lot of topics available to you. As Viola said, though, its important to be specific. The examples might actually still be a bit too broad, and you might want to focus on specific events/battles, but hopefully they will give you some idea of what you could study. As always, talk to your adviser to get suggestions about whether or not a particular topic is too broad/specific.
Hope that helps,
illu45