I believe that if you want to fly direct to Vietnam you will have only 2 options, HCMC and Hanoi. Firstly I would advise you to check the weather because it can vary considerably depending on where you are and this will obviously depend on what month your trip is panned for.
You will find French colonial architecture in both HCMC and Hanoi and lovely food, good clean restaurants and coffee shops. The coffee in Vietnam is by far the best I have ever tasted (I spent a few nights in HCMC and 10 nights on a separate occasion in Da Nang.
HCMC is a big city split up into several districts and I stayed in District 1 which was fairly central and I had a pleasant time although compared to Bangkok I do not think there is that much to see. The Saigon Skydeck is worth visiting for panoramic views of the city from the observation deck and the the Central Post Office is a lovely building. There are 2 so called Walking Streets, one is what you would expect, bars, street entertainment, hostels, restaurants and seedy massage parlours but the other is where you will find the statue of Ho Chi Minh and the central area is for walking only with water fountains, some lovely buildings and some street entertainment and music in the evenings.
All that said if the choice was between HCMC and Hanoi, I would probably choose Hanoi because it is possible to do a one day organised trip to Halong Bay from there and 5 to7 days may be a wee bit too long just to stay in HCMC or Hanoi. Now if you would consider getting a connecting flight I would definitely consider Da Nang. There you have the city and lovely beaches to relax and so much to do and see around and near the city. Hoi An is less than 1 hour away and the ancient city of Hue is also possible to visit. Nearer to Da Nang you will find Ba Na Hills, a beautiful scenic day out and also there is the Giant Lady Buddha and surrounding area with beautiful gardens and a fantastic view of Da Nang.
Once again please remember to check the weather for your destination for the month you will travel. For Vietnam I would advise you to check out the recent You Tube videos by Gio In The Philippines. He has just done a tour of some of the country including the 3 cities I mentioned.
Last thing - Vietnam is much cheaper than the Philippines for accommodation, food and getting around, something Gio mentions in a couple of his recent videos and I think they have now made changes to their visa policy so nationals of a lot of countries can get 90 days now without leaving (for a cost I think). I can see Vietnam becoming even more popular with Ex Pats now only a visa run is required every few months.
Seoul in South Korea is also worth consideration.