It being summer, I'd suggest taking this piece to the nearest large natural history museum or major university with a large zoology department. In your country, that is (see last paragraph). What you want is someone with a dugong skeleton on hand, to compare the piece with.
The issue is that most of it is so sculpted that it will be difficult to tell if it is a dugong vertebra (top of the back) or not, without that same vertebra to compare it in three dimensions (which willl present an opportunity for some great photos, incidentally). Oddly enough, museums tend to have such skeletons around, or to know who does have them. In my experience, if they're not too busy, they also enjoy puzzles like determining the source of a bone.
I'm a little puzzled, because dugong ribs (not vertebrae) have a reputation for being unusually dense. So far as I know, they act to ballast the animals in the water. Unfortunately, it's been handled so much that non-destructive DNA sampling will simply pick up human DNA.
As for legal status, yes, dugongs are in Appendix 1 of CITES, which is the most restrictive one.
Best,
F
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