Given that early in the season, when RB were still trying to compensate for the loss of the blown diffuser, Mark was regularly outperforming Seb yet Fernando was trashing Massa in their difficult car, I'm inclined to believe that Seb would have initially struggled.
Indeed, on that basis he would have been nowhere.
I have no doubt he would have got to grips with the car and performed to a similar level eventually though.
A Vettel in the middle of the pack would get involved in too many incidents. What stood out about Alonso this season was his consistency and maximum points return when all around him were going crazy. Vettel isn't cut from that cloth, he is impulsive and error-prone.
Looks like you keep picking a part of the season and drawing a conclusion from there, neglecting the majority of their times as partners for whatever reason. There are some parts in Alonso's seasons when he was beaten by his team mates: Trulli 2004, Massa 2012, and they're not as good as Webber IMO, especially Massa. Hamilton 2011 was beaten too, even almost all season long. But what does it make?
Nobody in this world has never been beaten by his team mate at any time in his career including the greats like Schumacher, Fangio, Senna, Clark, etc. You have to look at the whole time they were together as team mates, then draw a conclusion.