The acute in Spanish is a stress marker is it not, therefore (to this non-expert anyway) it makes more sense to treat the accented and unaccented vowels the same for collation purposes than would be the case for languages where the diacritic denotes a different phoneme. That said, I don't know enough Spanish to know whether the acute ever distinguishes separate words, which would perhaps weaken that argument.
The grave in Italian serves the same purpose AFAIK so would be interesting to compare the rules for that language.
Having said that, the standard collation rules are:
- First, unmarked vowels.
- Second, vowels with an acute accent.
- Last, vowels with diaeresis (the diaeresis is very rare and in native words only appears in the syllables "güe" and "güi", were it means that the U should be pronounced, as otherwise it would be silent).
The grave in Italian serves the same purpose AFAIK so would be interesting to compare the rules for that language.