Pep Guardiola was the most successful manager of Barcelona ever during his short four year stint at the helm of the greatest club in the world. When he decided to step down in the summer and take a year off, Pep was instrumental in his assistant Tito Vilanova taking over. The two have been friends since coming through the youth system at the club and worked together since the team was in the fouth division.
So when Vilanova was diagnosed with a recurrence of the cancer he had a year ago, Pep would only be thinking about his friend and colleague getting better, not about taking temporary charge of the team. There have been rumours and reports suggesting this, but there is no move from Guardiola. Everyone concerned with the club will be hoping and expecting Vilanova to receive the best medical care and get over it.
There is no way that Guardiola would be thinking of replacing him.