The vote is a constitutional change - as you mentioned, about how elected officials are selected.
The issue is that Mario Renzi (current PM) is backing this to the point he's said he'll quit if it doesn't pass. The banks (according to my husbands cousins) also really want this to pass.
Polls are showing that "no" will win... And Renzi will quit. This forces an early election - and the populism that was behind Brexit and our recent elections in the US are thought to, maybe, sweep some anti-EU folks into office. Once in... they might pull out of the EU or the Euro.
This would cause more friction between Germany (who holds much of Italy's debt) and Italy... and major stress in the euro bond market.
At least that's how I read it.
My husband's cousins say that Renzi is the only one pushing this brexit comparison and it's ridiculous.