The website is indeed very confusing. To get the online discount you must purchase your pass 7 days in advance. My online attempt failed, and since that made me wary of the process I decided to buy in person. Only cost about 4 euros more. You can buy the pass from an agent at VCE or the train station as well as a few other stops (Accademia) - or buy from a machine at any stop.
Just returned from Venice and found the 7 day Vaporetto pass a terrific value even tho I didn't buy it online. Did not buy a museum pass. - consider carefully whether you will visit enough of the sites to make it worthwhile.
Good luck