I agree with Tawny and think your query reads more like a synopsis. The story sounds good; if you can shorten it to a paragraph I think you will get some partial requests. When I was writing my query I read some query advice from agent Nathan Bransford. He wrote a mad-libs version of a query on his blog and it really helped me to summarize my query. Here's the blog link. http://blog.nathanbransford.com/2008/03/query-letter-mad-lib.html
I wanted to add about historical novels in the 50s and 60s. Read 'What I Saw and How I Lied' by Judy Blundell (Simon Pulse). That YA novel is set in the 50s and it won the National Book Award.
Don't worry about what's selling and what isn't. I went to a big writer's conference in '99 and the keynote speaker (an editor from a big pub house) said YA is dead, stop writing it, no one reads it. The Perks of Being a Wallflower came out and (MTV press published it because there WERE no publishers for YA) and everyone started scrambling for new YA voices. I think a book about the Viet Nam war would be great, especially with a boy character, which is so needed right now. I go to a lot of conferences and I'm always hearing more boys in YA please.
I think your query is too rambling and unfocused. This tells agents and eds that your book must be unfocused too. Even if it isn't.
I see you have your heart set on the thriller twist. I know the feeling. If you've ever seen "This Boy's Life" with Leonardo Dicaprio then you know, just like with the film October Sky how powerful just the events happening in these protaq's life were. They were going through things just about anyone could relate to (Robert D was scary good saying "Shut your piehole!"). Maybe something to think about is why your book starts off one way then turns into something else, and if the thriller part is the selling point. I mean, now on top of his parents getting killed the killer is after him (but I'm confused now. Is the boy who killed the protag's parents the same one that kills the girl and is after Mark? Or is that kid the one Mark falsely accuses, and its really another kid?). If you can pull it off, then wow. Keep us updated on your progress.