From a technical POV the site is designed really well. My problem is that approaching the site from a customer perspective it just seems like information overload. When a customer is coming to your website they already have an idea in their head of where they want to go and when and all they want to know is how much it's going to cost. If I goto your website with that in mind I cannot quickly get that. I have to find a small button on the left hand side that says "customize your trip".
I have a soft spot for travel websites cause I used to be a travel agent a long time ago and I helped them with their website back in the day. I get the sense from your website that you have a lot of information on travel to Vietnam but I don't get a feeling that you are selling trips to Vietnam. Let me give you another site as comparison.http://www.asiatravel.com/
On this site the first thing I see is a booking engine on my left hand side. It tells a customer, "Yes we sell travel, put in your information here and we'll sell it to you."
This site is using it's own booking engine. You may not have access to it but you can still arrange your website in such a way so that booking trips to Vietnam is more important to simply providing information about Vietnam. You can provide a small section with drop down menu's for destination, duration, and dates of travel. And when they click "submit" it goes to your trip-planner page with those sections already filled out.
You have a serious case of menu-overload. You have your top menu, the travel menu, and your sidebar menu. It makes the site look intimidating and not very user friendly. Simplify things. Use drop down menu's to cut down on the number of top level navigation links. For instance takeyour Destination Travel menu. Just make a top level item named Travel Information and put all of your destinations under it like this.
</ul></li><!--END Travel Information-->
This will allow you to consolidate everything into one area and it doesn't look overwhelming at first glance. Consolidate other menu items as well until you have a simple and ordered navigation structure that is easy for users to navigate through while at the same time providing them with all the in-depth information that you already provide.
The little flashing star icons are about 10 years outdated. What I would do personally would be to make an icon using the word "Special" but written in Vietnamese calligraphy. It would be easier on the eyes and would be IMO more hip and modern looking.
The only other thing I would suggest would be writing some kind of blog where you can highlight Vietnamese culture, events, food, travel information and other things. Why do this? Well for one it makes your website seem more personal and it will allow you to highlight Vietnam as a great place to visit, not as a great place to read about. Social networking tools like blogs are a great way to connect to a whole new customer base and gain a larger audience.
Note: Getting some high res pictures of Vietnam and using parts of them on your website as design elements can help as well.