Cena's actions and the way he portrayed himself with the gimmicks really got him popular, after the attitude era there were a lot of super stars but Cena had a different touch back then and apart from that he is built and really strong too. He was also good with the mic. All of these made a lot of people Cena fans. WWE then saw this opportunity and started promoting him as a face by winning major matches and defeating major opponents, this worked out too well as it pulled a lot of kids and family into WWE through Cena. He must have been a good performer as a rookie too, WWE team not only looks upon impressive moves and stage profile but also the ability of the superstar to do it safely.
Vince does not completely decide it, there is a huge team of creative writers who decides the storyline and characters, they saw the opportunity to generate more money and audience through Cena and they took it, it is just a business strategy. The ultimate objective is to pump cash, Cena was at the right place at the right time with the right kind of attitude and gimmicks. WWE business style is implicit, i.e. they improvise or modify as the show moves on. Even Vince's appearance and actions are influenced a lot by the creative team. All these showbiz factors lead to Cena being the face of the WWE, Cena although hated by a lot now should be given some credit in being able to carry it so far.
However now they have pushed it too far and he is one of the most contradicted superstar ever, his over hype is questioned by fans and legends as well. Hopefully it will change, the recent signs indicate a turn in Cena's career which again will be dealt with the strategy WWE writers have decided upon.