North, Central or South?! Solo female

What time of year and where are you starting from?
17 days is a reasonable amount of time for doing the north, central, and south. No reason you would have to exclude Ha Long Bay, considering it is relatively easy to get to and visit.

But if El Nido is on your Philippines itinerary, it completely blows away Ha Long Bay.

It will be from August 1st this year, and yes El Nido is definitely on my list!

the more I think about it I'm really drawn to spend more time venturing around Hoi An/ Hue region and see what places such as Dalat and Nha Trang have to offer but I'm open to other suggestions!/where to stay

I also don't want to rush the visit and thats why I'm feeling more inclined to stay in one area to explore the culture more.. and as you said- I've got the Philippines!

Halong Bay is really nice, but there are several other places in Asia that are equally nice, so I wouldn't say it's a must see. Wherever you are in Vietnam it's a great cultural experience. Hanoi is better than HCMC though, so I would go there and fly from there. Don't forget you will need to show onward ticket leaving the Philippines when you check in for your Philippines flight. Once I almost got stuck in Vietnam because I forgot to print it out. They really DO check and deny boarding!

As long as you avoid going to places in the mountains, you can easily do a north to south journey through Vietnam in 17 days. You'd start in Hanoi, and end in Ho Chi Mihn City, with time for Halong bay, Hoi An, Hue, a quick Mekong delta trip, and maybe another stop like Nha Trang along the way. You can use overnight buses to go between regions, or flights if you prefer.

All of the above - I didn't particularly like Nha Trang (it felt pretty soulless). But would definitely recommend Hoi An - I spent 3 days and could easily have spent a week there. Also I'd recommend at least one night homestay in Mekong Delta area, easily accessible from HCMC

WHICH MONTH ? Very important because of difference in climate, with "No-No months in each region)

In 17 days, the North + the center are perfect, the South being less interesting ; I would say

10 days North (Hanoi, Halong, Sapa, Bac Ha, ending with 3 days in deligthful Tam Coc)

Night of Day 10 ; night train NINH BINH-DONG HOI(no need to go back to Hanoi)

2 days PHONG NHA Grottoes

2 days Hue

One day BY CAR for the superb trip Hue-fishing village of Lang Co-Pass of the Clouds (buses go through a tunnel, you miss the stunning panorama north and south)-Cham Museum of Danang-Marble Mountain-Hoi An

3 days Hoi An with My Son

If OK for swimming, cut 2 days in the North and add themin Hoi An to spend some time in very charming Cham Island off Hoi An

I think you should go to from north to south. Let start at Hanoi , then go to Halong, next Hoi An , next some where you would like. And finish in Mekong Delta because it is easy to you through Vietnam to Philippines .Let research. Goodluck