How does Vietnamese writing work with the Romanized letters and standard spelling rules for English words?

Basically, because Vietnamese has some different vowels from English, there are some additional, modified vowel letters and markings to denote them. Vietnamese was also transcribed in its romanized form to include two “d’s”, one with a horizontal line through the vertical, denoting either an English “d” sound or a “y” or “z” sound depending on the dialect.

Additionally, Vietnamese has both an aspirated and unaspirated “t” sound, the first of which is denoted using “th”.

The tones are then overlaid on top of all this.