I'll list them in the geographical position from the North to the South order & my comment:
Beach (city/province) - Point (/10) - comment
======================================...
Do Son (Hai Phong)
5+
near Hanoi (the North, 120km), not many foreigners, it seems they're attracted by a Casino when choosing to go there

Cat Ba (beside, Hai Phong)
8
has national forest
Ha Long bay (Quang Ninh)
10
near Hanoi (130km) RECOMMEND - the Natural World Heritage, superb, magnificent
Cua Lo (Nghe An)
7+
cheap, beautiful, clean beach, you can combine to see the vestige of Ho Chi Minh president (when in childhood time)
Da Nang (Da Nang)
8+
another beautiful beach in the Top World List, RECOMMEND, near My Son (the holy land, world heritage)
Cua Dai (near Hoi An - ancient town, world heritage)
8+
RECOMMEND clean beach, white sand, simple & hospitable people, many places to go around, lot of backpackers
Lang Co (near Hue, 25km?!)
9
wow, a place for lovers, you will don't regret if u choose this one, don't miss this place

you can find your self here

many spectacular scences around here (Hai Van mountain pass...)
Nha Trang bay & its coral islands (Nha Trang)
9+ RECOMMEND
very interesting, in Top Beaches List of the world, lot of backpackers
Mui Ne (Phan Thiet)
9 RECOMMEND
attractive place, sand dune's very stunning, the heaven of resorts (not expensive

pure air
Vung Tau (Ba Ria - Vung Tau)
9 RECOMMEND
near Saigon (Ho Chi Minh City), the beach sloped gently, clean & pure
Con Dao (Vung Tau)
8++ Recommend
very natural & interesting
Phu Quoc (Kien Giang) island
9++
a place for honeymoon is not a bad choice

quaint, wonderful, many things to discover
Other beaches:
Ca Na, Vung Ro (Phu Yen), Tay Ninh...well uncountable (in the middle of Vietnam), all of them are very attractive 'cos they still haven't been affected by human, very natural, it seems travel industry doesn't impact much on them, friendly people, many gorgeous landscapes (of course, RECOMMEND

.
Some prices in general:
bus (open tour) Hanoi-Hue, Hue-NhaTrang, NhaTrang-Saigon: about 10-15$, cheap & easy to buy ticket (one price for local or foreigners).
Ferry: ~1-2$, not much
Ship: 5-10$ depend on the distance
Motor bike: 6-8$/day
Local food: very delicious, cheap, fresh & plentiful (ex: seafood...)
Hope some info's helpful for you
See you in Vietnam - the hidden charm, a friendly & safe destination :-)