One of the most common exclamations: ủa, is expressed when something mismatches your expectation.

Ví dụ / Example
🇻🇳 Ủa? Mới tới hả?
🇺🇸 Huh? You just got here?

Here, someone arrived at a different time than expected.

Ví dụ / Example
🇻🇳 Ủa? Tưởng không tới chứ!
🇺🇸 Huh! I thought you weren't coming!

Here, someone arrived late; you expected them not coming.

Ví dụ / Example
🇻🇳 Ủa. Đâu rồi?
🇺🇸 Oh. Where is (it/he/she)?

Here, you expected something/someone to be somewhere. Maybe you thought a key was under a book. Or you got out of the bathroom at the mall and your cousin is gone.

Surprises: they imply something didn't match your expectation. "Ủa" is usually used to express surprises too.

Ví dụ / Examples
🇻🇳 Ủa! Nhi với Lan biết nhau hả?
🇺🇸 What! You two (Nhi & Lan) know each other?

🇻🇳 Ủa. Làm ở đây hả?
🇺🇸 Oh! You work here?

ủa — expresses mismatched expectations or surprises

So that covers the normal usage of the word. However, tone of speech can change a lot. Recall the example above:

Lặp lại / Recall
🇻🇳 Ủa? Tưởng không tới chứ!
🇺🇸 Huh! I thought you weren't coming!

This expression above, whether in English or Vietnamese, has different meanings when said in a casual tone versus serious tone.

Ví dụ / Example
🇻🇳 Ủa? Làm một mình được mà!
🇺🇸 Oh! But you could do it by yourself!

Earlier, person X offered person Y help but Y declined. Now Y came back and asked for help, and this is what X might say, tongue-in-cheek.

In this case, you could think it's the mismatched expectation of someone else that the speaker is using "ủa" for.