I personally think that Sonia's current attire is where it should be and here are my reasons:
A. She's a woman so she can get away with it.
B. Since females of the species are generally not as strong as men (unfortunately) (but they can be nowadays), Sonia's not going to wear a whole lot of armor that would weigh her down especially when fighting vampires who're fast on their feet. When it comes to being a woman, speed is you're greatest advantage. There are also other Belmonts in the clan who're wearing a lot less the she is so I'd say her outfit is in a comfortable middle.

-X