Partially correct. Rogue consulates will still approve such applicants if the special fixer fee is paid on top. Same for immigration blacklist removal via 'lawyer', etc. But that's in-country. Everything has a price.
Your agent didn't pay enough to the office. Prices for 'guaranteed approval' have gone up recently due to increased demand. We're also seeing more DIY denials to encourage people to pay an agent. But some agents are trying to keep too big of a cut and Vietnam consulate boss isn't happy with it.