Vietnamese train stations often consist of little more than a ticket office and a waiting room which many European travelers will find quite bare and basic in terms of amenities (for American travelers it will not be all that different from your average Amtrak station). Recently free wifi has been deployed to most major stations and ticket QR codes are often being scanned directly from smartphones, so the process is rapidly modernizing.
With the exceptions of Hanoi and Ho Chi Minh City, few stations have food or convenience stores inside but you should be able to find anything you need in the area directly surrounding the station. Trains stop only shortly at most stations so your chances to purchase items once onboard will be limited. The railway might have affordable food and drinks.