In most natural contexts, đã is formal. A Vietnamese would formally say "Anh ấy đã chết trong chiến tranh" ("He died in the war"), but not casually say *"Cháu đã ăn cơm" ("I had a meal") or *"Hôm qua tao đã thấy nó ở đường Yersin" ("I saw him yesterday on Yersin Street"). Instead, they would say "Cháu ăn cơm rồi" and "Hôm qua tao thấy nó ở đường Yersin".
Otherwise, it is used artificially in common awkward translation work; or casually in very few patterns only for emphasis, and in these cases, it can be considered equivalent to the use of the English present perfect tense. For example:
Con đã ăn cơm chưa?
Have you eaten yet?
Mày có biết bờ biển Nha Trang đẹp thế nào hông? ・ Tao đã đi Nha Trang bao giờ đâu mà biết!
Do you have any idea how beautiful the Nha Trang beaches are? ・ How am I supposed to know? I've never been to Nha Trang!