Such polls don't mean much, esp. in a tabloid that doesn't even give its source, but even if it is legit, a large number of the the 74.7% of people who voted for Vettel may not even be F1 fans - at least not to the extent that they would go to a F1 race or regularly watch races on TV. While maybe a larger percentage of people from Schumacher's 68.5% may have been more hard core F1 fans.
Also take into consideration that Vettel has the advantage of being a current driver.
In other words Vettel may be more popular with the general public for reasons that you yourself mentioned - eg. "my grandma likes him because he looks nice and attractive" but Schumacher might have had a larger hard core following from a demographics that actually matters from the aspect of viewer ratings and race attendance. In fact, I do think that's the case. I feel Germans were more passionate about F1 during Schumacher's time.
It were different times... people used to be more in cars back then and TV, it was before Internet got as widespread. There were videogames, but they weren't played by every kid/teen as they are today... I was one of the nerds who did play games of course. Also F1 used to be alot more affordable back then. German populace in general more wealthy. Less people without job... all those things matter....