Tôi đã trả lời tin nhắn của Lan rồi, nhưng An vẫn chưa trả lời tôi.

They often overlap, but they do slightly different jobs.

• đã marks the action as having happened: Tôi đã trả lời... = I replied / I have replied...
• rồi adds the sense of already, or that the action is completed.

So:

• Tôi đã trả lời tin nhắn của Lan rồi = I’ve already replied to Lan’s message.

In everyday Vietnamese, speakers may use:

• đã ... rồi
• just đã
• just rồi

depending on emphasis and style. Using both together is very common and sounds natural.

vẫn chưa means still not yet.

So:

• An vẫn chưa trả lời tôi = An still hasn’t replied to me yet

The parts are:

• vẫn = still
• chưa = not yet

Together they show that the speaker expected the reply to happen by now, but it has not happened.

Vietnamese does not work like English tense systems. It usually does not change the verb form for tense.

Instead, Vietnamese often shows time and aspect through:

• time words
• particles such as đã, đang, sẽ, rồi, chưa
• context

So trả lời always stays trả lời. The time meaning comes from words like:

• đã = past/completed
• rồi = already/completed
• chưa = not yet

Trả lời is a very common verb meaning to answer / to reply.

Even though it is written as two syllables with a space, it functions as one verb phrase.

Examples:

• trả lời câu hỏi = answer a question
• trả lời tin nhắn = reply to a message
• trả lời tôi = reply to me

In this sentence, it appears twice:

• trả lời tin nhắn của Lan = reply to Lan’s message
• trả lời tôi = reply to me

It means Lan’s message or the message from Lan / belonging to Lan, depending on context.

• tin nhắn = message / text message
• của = of, belonging to
• Lan = Lan

So tin nhắn của Lan most naturally means:

• the message that belongs to Lan
• the message from Lan
• Lan’s message

It does not normally mean a message to Lan. If you wanted that, Vietnamese would express it differently, such as:

• tin nhắn gửi cho Lan = a message sent to Lan

Because tôi is the object there.

Vietnamese basic word order is usually:

• Subject + Verb + Object

So:

• An = subject
• trả lời = verb
• tôi = object

That gives:

• An vẫn chưa trả lời tôi = An still hasn’t replied to me

This is similar to English An hasn’t replied to me, where me comes after the verb.

Sometimes yes, but not here if you want to keep the meaning clear.

• An vẫn chưa trả lời would mean An still hasn’t replied / answered, but it would not say to whom.
• An vẫn chưa trả lời tôi clearly means An still hasn’t replied to me.

Vietnamese often drops pronouns when context makes them obvious, but including tôi here is normal and useful.

Nhưng means but.

It connects two contrasting ideas:

• I already replied to Lan’s message
• but An still hasn’t replied to me

So the contrast is:

• one reply has already happened
• the other reply has still not happened

Yes. In many contexts, trả lời can be translated as either answer or reply.

For example:

• trả lời câu hỏi = answer a question
• trả lời email = reply to an email
• trả lời tin nhắn = reply to a message
• trả lời tôi = answer me / reply to me

In this sentence, reply sounds more natural in English because the context is messages.

Vietnamese often expresses this idea without a separate preposition.

English says:

• reply to me

Vietnamese says:

• trả lời tôi

So the person receiving the reply can come directly after the verb as the object.

Likewise:

• trả lời Lan = reply to Lan
• trả lời giáo viên = answer the teacher
• trả lời câu hỏi = answer the question

This is just how the verb works in Vietnamese.

Usually yes, or at least some kind of message.

• tin nhắn most commonly means text message or message

Depending on context, English translations could be:

• message
• text
• text message

So trả lời tin nhắn của Lan often means:

• reply to Lan’s text
• reply to Lan’s message

Yes.

All of these are possible:

• Tôi đã trả lời tin nhắn của Lan rồi.
• Tôi trả lời tin nhắn của Lan rồi.
• Tôi đã trả lời tin nhắn của Lan.

The full version with đã ... rồi strongly shows that the action is completed and already done.

Very roughly:

• đã focuses on past/completion
• rồi focuses on already/completed state

Using both sounds very natural in conversational Vietnamese.

Vietnamese usually does not mark the subject with special endings or cases. You understand the role from word order and context.

In:

• An vẫn chưa trả lời tôi

the pattern is:

• An = subject
• vẫn chưa trả lời = verb phrase
• tôi = object

So An is understood as the one who has not replied.

This is one reason word order is very important in Vietnamese.

No. Chưa can be used by itself.

Compare:

• An chưa trả lời tôi. = An hasn’t replied to me yet.
• An vẫn chưa trả lời tôi. = An still hasn’t replied to me yet.

Adding vẫn makes the feeling stronger: the speaker is emphasizing that the expected reply is still missing.

Yes. Sentence-final rồi is very common in Vietnamese.

It often signals:

• completion
• change of state
• already

Examples:

• Tôi ăn rồi. = I already ate / I’ve eaten already.
• Xong rồi. = It’s done already.
• Muộn rồi. = It’s late already.

In your sentence, rồi helps show that the reply to Lan is already completed before the contrast with An’s non-reply.