No it's not. Phrasal verbs in English are literally multiple words joined together, with a new meaning derived from those constituent words. Granted, the Wiktionary page I linked is not direct evidence because that is a list of terms, not words, but you can find many polysyllabic words in it if you look.

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."