First of all, welcome to FT!

Since Easyjet has a direct NAP-VIE I'm assuming your question is about two completely disconnected flights.

In general for an intra-Schengen flight (such as your NAP-ATH and ATH-VIE) two hours before departure is enough. Make it 2.5h if you're flying on high holiday season.

As far as getting your bag, it really depends. I'd say 30 minutes after arrival is probably not too far from the truth (by the time you disembark and walk to the exit it won't be much of a wait anyway).

If on the other hand you booked a "connection" on Easyjet, please report your flight numbers and times, as well as whether this is on one ticket or not. If not, I wouldn't touch it with less than 3-4h between flights.