It's explicitly said that Vitra was among the weakest but most versatile of the Dragon Kings. Saji is basically the Dragon Kings' version of Kiba. Not the strongest in outright fighting, but hello, he's got skills to handle tons of situations.
As for Xenovia, she really needs to master her sword. Frankly if she does that and gets some good skills? That's all she will need. Ex-Durandal is frankly scary with the number of tricks it can do.