Thanks.
It's Vietnamese. So for example last name Nguyễn is how it's printed on Passport, so between Nguyễn and Nguyen we're not sure what we should put in IMM5406 and what on LOE.
I was guessing likely Vietnamese. You'll have to use your judgment. My guess - not knowing the language - is that this is not that big a deal. If there are no common versions of names with different diacritics, or they're not considered 'different' names, it won't ultimately matter much - probably.
Or in your example - as far as I'm aware - it's always written Nguyễn (or Nguyen 'for foreigners') and it's not like half the country uses Nguyen and the other half Nguyễn and see them as different names.*
On the other hand it won't take much time for you to write these out on a separate sheet of paper and add it just in case.
If the passport is machine readable, at the bottom it will have the name without the diacritics in those codes. That's what I was referring to. Canada will - for purposes of PR docs and the like, presumably including their internal systems - ignore the diacritics that we don't use (eg that aren't used in French, basically).
*Digression: Whereas although Cyrillic is usually quite phonetic in the languages that use it, the transliterations into English have changed over time and sometimes preference - or via other languages with their own conventions - leading to considerable variety of versions of the same name, eg Gorbachev/Gorbachov/Horbachov/Gorbachoff/Gorbatchiev/Gorbatchioff or even Gorbacov. (The standard English version - Gorbachev - is outright wrong in the sense that the last vowel is a long o ... but that's what we ended up with.) Sometimes these different transliterations can even mean family names starting with different letters or digraphs - Ch instead of Sh-.
Given names that we think of as the same or from the same root have slight variations that are considered different, even though would end up transliterated identically into English. And that's before the fact that Ukrainian, Belarusian and Russian share names (which might notionally be seen as 'ethnic' names regardless of where the individual lives or what language they speak) but transliterate/pronounce them quite differently, and those different transliterations (alas) have been politicised in some contexts (eg Vladimir vs Volodymyr). So they do want the 'original' name in native script (which will vary by country), even if that's not perfectly consistent either.
Sorry, this is a digression.