Two diacritic systems layered. Vietnamese distinguishes meaning using both vowel quality (which vowel is it?) and tone (what pitch pattern?). The writing system reflects this with two independent marks that can combine on a single letter.
The six tones. Vietnamese has six lexical tones, five of which are marked with a diacritic (the sixth, the mid-level tone, is unmarked). Ngang (level, unmarked): ma. Huyền (falling, grave): mà. Sắc (rising, acute): má. Hỏi (dipping, hook): mả. Ngã (broken, tilde): mã. Nặng (heavy, dot below): mạ. Six completely different words, distinguished only by tone.
Vowel marks modify vowel identity. Vietnamese has 12 distinct vowel sounds written with 12 different letter forms: a, ă, â, e, ê, i, o, ô, ơ, u, ư, y. The circumflex (â, ê, ô), breve (ă), and horn (ơ, ư) change which vowel you're reading.
Stacking means a character can have two diacritics. Ế is ê (vowel mark: circumflex) + acute tone. Ấ is â + acute. This produces a large character set — about 134 unique vowel+tone combinations. Windows-1252 only covers ~20 of them (the ones also used in French, Portuguese, etc.). The rest require Unicode input.
Method: TELEX and VNI input schemes. Vietnamese users typically install Unikey or a similar input method editor. You type regular letters plus a code letter: aa = â, aw = ă, dd = đ, as = á, af = à. For occasional use, copying from this page is faster.
Đ / đ is a separate letter. The crossed-d (đ) represents a sound different from regular d in Vietnamese. It's a distinct letter of the alphabet, not an accent. Đ = U+0110, đ = U+0111.