It's great living next to Land of Cheap aka The World's Workshop aka China
It is now possible to buy a half-decent feature cell handset, with two SIM slots, for $15 in Ho Chi Minh City. Bottom end Android smartphones, with capacitive screens, go for $35.
Couple these prices with VietNam's government-mandated open (unlocked) cell handset policy and VNPT's basic cell service it's easy to see why students of all ages carry some form of cell handset.
The government-owned, for profit, VNPT basic plan supplies a starter prepaid SIM and airtime for $10 and coupled with their 'poor peoples plan' (one outgoing call/SMS per month to maintain SIM validity) can keep people available for over a year. I have a SMS only unit on which I dropped $20 six years ago using the 'poor' plan and I can still receive incoming SMS messages and have a balance of Dong 210,000 (USD$9.96 / GBP6.08).
A replacement SIM costs $0.15 + a Dong10,000 (USD$0.47 / GBP0.29) top up. VNPT still makes millions (dollars) profits. Their subsidiary, Mobiphone, caters to the youth market with all sorts of messaging, etc AND FREE EVENINGS/OVERNIGHTS!
Samsung, and Apple, maintain their prices for high end cell handsets - Samsung used it's regional activation feature to 'eliminate' the grey market - although all Samsung units are sold unlocked in VietNam. As VietNam has no laws against price fixing, and legit dealers have to sell at stipulated prices, they give 'promotions' (aka discounts) by loading you down with very useful accessories such as Bluetooth headsets, battery chargers, etc.