This is like arguing phrasal verbs are actually one word.
Many Vietnamese words are indivisible. E.g. "chuẩn bị", which comes from Chinese "zhǔnbèi". It was borrowed as a complete 2-syllable word. Any further division would cause it to lose all meaning.
Consider the inverse: if you decided to create a new spelling convention where every syllable in English were written spaced apart, then a word like "tomorrow" would become "to mo rrow". Does that mean that "mo" is now a separate word? What does it mean? If I looked up "mo" in a dictionary, what would I find?
If you're still unconvinced, the second paragraph on the Wikipedia page for Vietnamese directly addresses this: "Although it is often mistakenly thought as being an monosyllabic language, Vietnamese words typically consist of from one to many as ten morphemes or syllables; the majority of Vietnamese vocabulary are disyllabic and trisyllabic words."
But I'll throw it back at you:
https://en.m.wikipedia.org/wiki/Vietnamese_morphology#:~:tex....
"Vietnamese is often considered to be monosyllabic as its morphemes are considered to be monosyllabic e.g. "tim" meaning "heart". However, some Vietnamese words may consist of one or more syllables, composed of monosyllabic morphemes that form together to create another word. An instance of a compound word "mạnh mẽ" is derived from morphemes mạnh meaning "strong", mẽ meaning "dramatic", fused together to create the word mạnh mẽ to mean "powerful".
There is a general tendency for words to have one or two syllables. Words with two syllables are often of Sino-Vietnamese origin. A few words are three or four syllables. A few polysyllabic words are formed from reduplicative derivation."
Basically - monosyllabic.
Is it a beautiful day where you are? You should go outside and enjoy it:)
I know other Vietnamese speakers who also believe the language is monosyllabic. Honestly, speaking the language is a disadvantage for you here. It's like how most Chinese speakers think that other Chinese languages are dialects and not languages. Or how native English speakers will confidently say that "upward" is a preposition. Speaking a language does not give you insight into its linguistics, only a false sense of confidence about it.