Chợ đêm rất đông vui.

A natural breakdown is:

• chợ = market
• đêm = night
• chợ đêm = night market
• rất = very
• đông vui = crowded and lively / bustling

So the structure is:

• Chợ đêm = the night market
• rất đông vui = is very lively and crowded

In Vietnamese, chợ đêm is a noun phrase, and đông vui describes it.

In Vietnamese, you often do not need a verb like to be when describing something with an adjective.

So:

• Chợ đêm rất đông vui
  literally works like
• Night market very crowded-lively

This is a normal Vietnamese sentence pattern:

• Noun + adjective
• Noun + rất + adjective

You do not need là here. Using là before adjectives in this kind of sentence would usually sound unnatural.

Rất means very. It goes before the adjective or descriptive phrase.

So:

• rất đông vui = very crowded and lively

This is the normal order:

• noun + rất + adjective

Examples:

• Trời rất đẹp = The weather is very nice.
• Quán này rất đông = This place is very crowded.

So in your sentence, rất correctly comes before đông vui.

It is two words, but they often work together as a very common descriptive pair.

• đông = crowded
• vui = fun, cheerful, lively

Together, đông vui describes a place that has many people and a lively atmosphere. English often translates this as:

• busy and lively
• crowded and cheerful
• bustling

So even though it is made of two words, learners will often see it used almost like a set expression.

Sometimes yes, in translation, but not exactly.

Busy in English can simply mean there is a lot of activity.
Đông vui usually adds a positive feeling:

• many people are there
• the atmosphere feels lively
• it feels enjoyable, energetic, or festive

So đông vui is often more positive than just crowded. A place can be đông without being vui, but đông vui suggests both.

In Vietnamese, the main noun usually comes first, and the describing noun comes after it.

So:

• chợ đêm = market + night = night market

This is very common in Vietnamese compound nouns:

• xe đạp = bicycle
• trà đá = iced tea
• cà phê sữa = coffee with milk

So chợ is the main thing, and đêm specifies what kind of market it is.

Vietnamese normally does not use articles like a, an, or the the way English does.

So chợ đêm could mean:

• a night market
• the night market
• night markets

The exact meaning depends on context.

In your sentence, English may translate it as The night market is very lively and crowded or Night markets are very lively and crowded, depending on the situation. Vietnamese often leaves that information unstated unless it needs to be made clear.

No, that word order is not natural.

The normal pattern is:

• Chợ đêm rất đông vui

In Vietnamese, rất usually comes before the adjective, not after it.

So:

• rất đông vui = correct
• đông vui rất = incorrect

Yes. In this sentence:

• Chợ đêm is the subject
• rất đông vui is the predicate, describing the subject

Vietnamese often forms descriptive sentences without a separate copula like is. So the adjective phrase itself functions as the main description of the sentence.

A rough pronunciation guide is:

• Chợ ≈ chuh with a short, heavy falling tone
• đêm ≈ dem or dyem depending on accent
• rất ≈ zut or rut with a sharp rising tone on the vowel, ending in t
• đông ≈ dohng
• vui ≈ voo-ee blended quickly

The tones matter a lot:

• chợ has a heavy tone
• rất has a rising tone
• đêm, đông, vui are level in spelling, though actual pronunciation can vary by region

If a learner says the words without tones, people may still guess the meaning from context, but using the correct tones makes a big difference.

By itself, it is usually understood as a present-time statement or a general description.

• Chợ đêm rất đông vui = The night market is very lively and crowded.

Vietnamese does not mark tense the same way English does. If you want to make the time clearer, you would add time words or markers, for example:

• Hôm qua chợ đêm rất đông vui = The night market was very lively and crowded yesterday.
• Tối nay chợ đêm rất đông vui = The night market is very lively and crowded tonight.

So the basic sentence does not explicitly mark tense.

Yes. Depending on tone and style, you could use other words such as:

• quá = too / very
• lắm = very
• cực kỳ = extremely

Examples:

• Chợ đêm đông vui lắm = The night market is very lively.
• Chợ đêm đông vui quá = The night market is so lively / very lively.
• Chợ đêm cực kỳ đông vui = The night market is extremely lively.

But rất is one of the most neutral and standard choices, so it is great for learners.

Sometimes you can separate them in a longer sentence, but in this sentence they naturally go together.

For example, you might hear:

• Chợ đêm rất đông và vui = The night market is very crowded and lively.

That is understandable, but đông vui as a pair sounds smoother and more natural in many everyday contexts.

So for this sentence, rất đông vui is the best and most idiomatic form.