Actually, I think 17 is the only number whose Roman numeral is an anagram for 'vixi'. Think about it: if you have one x, one v, and two i's, you've got to start with the x, because it represents the largest number. Sure, you have ix for 9, but not ixv for 14, which is written xiv. The general rule is that only one i can be placed before a larger number to indicate subtraction. In other words, you rarely (never?) see xiiv for 13, which is conventionally written xiii. Can we arrange our letters xivi? No, because that implies one less than than fifteen plus one. Which is 15, written simply xv. So if we have to start with the x, and we can't write xiiv or xivi, we're left with a single number that can be written with the four Roman numerals: xvii, or 17.

My own close studies of Italian literature and culture have revealed an Italian cultural affinity for the anagram, the acrostic, and other kinds of visual/spatial word play that can be found in medieval versi intexti, in Dante's Divine Comedy, in the Futurist "Parole in libertà", in Umberto Eco.

I think it's unlikely that Italians would move from word to number, inferring 17 from "I lived", rather than the other way around. Standing in front of a grave, you think, "graves are creepy!" and not "hey, 'vixi' looks like 17, which reminds me of graves, and graves are creepy!" More likely: when you're faced with the number xvii in a non-cemetery context, you might think, "hey, looks like what I see on graves! creepy!"

Insomma, I think the "xvii as anagram for 'vixi'" theory is quite solid, as is the "17 is unlucky because it reminds the superstitious of graves and death" theory.