Vai và cơ bắp khá đau.

Vietnamese often does not use a verb like to be before adjectives or states.

So:

• vai và cơ bắp = shoulders and muscles
• khá đau = quite sore / hurt quite a bit

Literally, the structure is more like:

• Shoulders and muscles quite sore

This is normal Vietnamese grammar.
In sentences like this, đau acts like a stative verb/adjective, so you do not need là or any equivalent of are.

Vai means shoulder, but in Vietnamese, body-part nouns often do not have to show singular/plural as clearly as in English.

So vai can mean:

• the shoulder
• shoulders

depending on context.

In this sentence, English would usually translate it as shoulders, because the whole idea is about bodily soreness in general.

Cơ bắp means muscles or musculature.

It usually refers to muscles in a general sense, not necessarily one specific muscle.
So in this sentence, vai và cơ bắp means something like:

• shoulders and muscles
• the shoulders and muscle areas
• the body’s muscles

A related word is cơ, which can also mean muscle, but cơ bắp is a very common everyday expression.

Khá means quite, rather, or fairly.

So:

• đau = hurt / sore / painful
• khá đau = quite sore / fairly painful

It softens the statement compared with something stronger like:

• rất đau = very painful
• đau lắm = hurts a lot

So khá đau suggests a noticeable level of pain, but not necessarily extreme pain.

It can be thought of as either, depending on how you explain Vietnamese grammar.

In practice, đau means:

• to hurt
• to be painful
• to be sore

So in Vai và cơ bắp khá đau, it describes the condition of the shoulders and muscles.

English forces you to choose between something like:

• My shoulders and muscles hurt
• My shoulders and muscles are quite sore

Vietnamese does not make that distinction in the same way.

Vietnamese often leaves out possessives like my, your, or his/her when they are obvious from context.

If someone is talking about how they feel, vai và cơ bắp khá đau will normally be understood as:

• My shoulders and muscles are quite sore

If you want to make it explicit, you could say:

• Vai tôi và cơ bắp tôi khá đau

But that sounds repetitive and less natural in many contexts. Vietnamese usually prefers omission when the meaning is clear.

The basic order is:

• Vai và cơ bắp = subject
• khá = degree word
• đau = state/condition

So the pattern is:

noun phrase + degree word + state

This is very common in Vietnamese:

• Trời khá lạnh. = The weather is quite cold.
• Tôi khá mệt. = I’m quite tired.
• Vai và cơ bắp khá đau. = Shoulders and muscles are quite sore.

Yes, that is the most natural reading.

It means:

• the shoulders hurt
• the muscles hurt

So và simply joins two nouns:

• vai = shoulder(s)
• cơ bắp = muscles
• và = and

Together: shoulders and muscles

Not really as the main meaning.

Vai và cơ bắp sounds like shoulders and muscles as two coordinated items, not specifically shoulder muscles.

If you wanted to say shoulder muscles, you would need a more specific expression, such as something referring to muscles in the shoulder area.

So here, it is better understood as:

• The shoulders and the muscles are quite sore

not

• The shoulder muscles are quite sore

Yes, it is understandable and natural enough, especially in casual speech.

That said, Vietnamese speakers might also say things like:

• Vai và cơ bắp hơi đau. = My shoulders and muscles are a little sore.
• Vai và người tôi khá đau nhức. = My shoulders and body/muscles are quite achy.
• Vai tôi khá đau. = My shoulders hurt quite a bit.

So the given sentence is fine, but there are many natural ways to express body soreness in Vietnamese.

Đau is the basic word for hurt, painful, or sore.

Đau nhức is more specific and often suggests an aching, throbbing, or persistent soreness, especially in muscles, joints, shoulders, back, and so on.

So:

• khá đau = quite painful / quite sore
• khá đau nhức = quite achy / quite sore in an aching way

For muscles and shoulders, đau nhức is often very natural.

Yes. Common alternatives include:

• hơi đau = a little sore
• khá đau = fairly / quite sore
• rất đau = very painful
• đau lắm = hurts a lot

These words change the intensity.

For example:

• Vai và cơ bắp hơi đau. = My shoulders and muscles are a little sore.
• Vai và cơ bắp rất đau. = My shoulders and muscles hurt a lot.

Vietnamese usually does not need classifiers or plural markers for general body parts in a sentence like this.

Plural is often understood from context.
You do not need something like English shoulders with -s.

Also, classifiers are mainly used in counting or specifying nouns, for example:

• một cái vai = one shoulder

But in a general statement about pain or soreness, you simply say:

• vai
• cơ bắp

without classifiers.

You could say:

• Sau khi tập thể dục, vai và cơ bắp khá đau.
• Sau khi tập, vai và cơ bắp khá đau.

This means:

• After exercising, my shoulders and muscles are quite sore.

Adding time or cause like this is very common and often makes the sentence feel more complete in real conversation.