Hi- I'm debating whether to use tour agents to show me around the cities or to do it myself. I'm willing to make expenses but only if it's worth it.
I've been communicating with Threeland Sales travel agency. Does anyone know anything about them?
Am I going to miss on a lot if I go to all of these historic sites in Hanoi, Hue, Hoian if I don't have a travel guide?

imo, no, if you have a good travel guide book, but often a local person can make fast work of sightseeing if you have limited time....they know all the tricks, the best places to stand, etc...

I agree that reading ahead of time and a good guidebook with you and you don't need a guide. I didn't use a guide at all in VN.

Hi- thanks for the replies.
Just one more question: Do you recommend taking a open bus tour from Hue to Hoian and stopping at all the scenic sites or getting a private car. Any advice on these open bus tours?

You have so little time, I'd hire a private car as it will give you much more flexibility.