Want to split up my 3 weeks in vietnam , HCM and ??????

I'd say 1.5 weeks in either of those places is too long. Maybe you should plan for about 5 nights in HCMC and skip Vũng Tàu, which is kind of a nice place to live but doesn't have much to offer international tourists.

As for other beaches, you can look into Phu Quoc  island, Con Đâu island, Nha Trang, Quy Nhơn and Đà Nẵng/Hội An.

Hopefully, someone can add more specific info.  I actually haven't travelled that much in VN.

You should mention when you plan to visit.

thanks  a lot, I never heard of phu quoc island until you mentioned it now

it looks super nice

Im traveling in jan 10 to feb 1st

Brown water is beautiful. The Mekong River is brown. Feeds the whole country.
I haven't seen brown water at the beach.
Saigon 4 days
Phu Quoc 3 days, probably get bored tho
Vung Tau 4 days, you will appreciate after Phu Quoc
Nha Trang 4 days  OR
Da Lat 4 days (but maybe too cold and no beach)
Da Nang/Hoi An 5 days inc travel time

My suggestion is as following (including travelling time):
1.5 weeks in the North including Hanoi and other provinces such as Quang Ninh, Dien Bien ...;
6 days in Hue, Da Nang and nearby provinces;
6 days in HCMC and other provinces in Mekong Delta;
2 days in Da Lat.

Hope you enjoy yourself