Why is Vincentio upset when he discovers the imposters?

In act 5, scene 1, Vincentio discovers that another man is impersonating him. The merchant claims that he is Vincentio and calls for the true Vincentio's arrest. A little later, Vincentio sees Tranio dressed as Lucentio. This enrages Vincentio because he believes that Tranio has murdered Lucentio and appropriated Lucentio's identity, clothing, and money for his purposes.
So, Vincentio is upset when he discovers the imposters because he thinks that his son, Lucentio, has been murdered. This isn't true, of course. The real Lucentio soon reveals himself before matters come to a head; he asks his father Vincentio for forgiveness and also confesses to Baptista that he has married Bianca (Baptista's daughter).
Meanwhile, Lucentio asks Vincentio to forgive Tranio for disguising himself as his young master. Lucentio maintains that Tranio acted upon his command; so, Lucentio implores Vincentio to forgive Tranio for his sake.