I lived there very recently. Benzos are really not that easy to get anymore. If you had no idea where to go, I reckon you'd have to ask at 20 or 30 different pharmacies before you found one that stocked the stuff. Still, dodgy pharmacies still exist, and sometimes the ones that don't look dodgy are amongst the worst. I can think of a large, modern looking (by Vietnamese standards) pharmacy with tonnes of customers that sold OTC several different benzos in quantity, as well as Stilnox and Zopiclone. Nothing like Xanax or Klonopin, but most of the classical benzos and a few others. Strangely enough, not Diazepam. Go figure.
Yep Ketamine's dead easy to get due to its veterinary applications. The Vietnamese government has yet to catch on to its recreational potential, although I'd say it's only a matter of time.
Don't agree about the opiates - 30mg Codeine/500mg paracetamol is available in massive quantity at practically every pharmacy in North Vietnam (didn't spend so long in the South) it costs next to nothing, it's used widely and will be sold to you without a second glance. You can also buy French made 15mg codeine tablets which don't contain paracetamol.
And yes, heroin is ubiquitous (as you would expect considering proximity to the Golden Triangle). As a "rich foreigner", I wouldn't even know where to start looking for it. Not that I'd be interested in the stuff - in Vietnam it's mostly used by broke loser men* who usually but not always live in the mountainous regions. It's cheap as fuck because the vast majority of buyers are brokeass farmers in minority communities.
I know less about opium, didn't come across it in Vietnam nor hear anything about it (I don't think smackheads would be interested in opium when there's so much heroin about) but I came across a lot of opium in Lao when I went visiting.
Dope availability can be patchy in Vietnam - not my thing, but a few of my mates who were regular buyers (from major green dealers in the North, not smalltime amateurs) had to endure a few months-long weed droughts over the past couple of years.
*truly - these guys are so often fucking useless pricks. Their long suffering wives raise the family, generate its income alone (much of which gets shoved up the arm of their husband) and put up with their often violent junkie husbands, not to mention the risks associated with sharing needles. Countless tragic stories, hundreds of thousands of innocent lives ruined and/or cut short. It's a major problem.