They both speak basic in ordinary way unlike Yoda, but Yaddle spoke the same way as Yoda, so I guess Yoda's talking style has little to do with his race?
Having Yaddle talk like that was idiotic. I just assume she was a shameless copycat, in more ways than one.
Neither speech patterns or accents have any biological tie to race. Instead, they're a product of culture, the result of what an individual is exposed to and thus learns to imitate while mastering language. The likeliest reason for the difference in speech patterns between Yoda and Yaddle, and Vandar and Oteg, is a difference of where they grew up and around whom they learned to speak. It could also be a conscious decision on the part of Vandar and Oteg to forgo their people's style of speaking in favor of the one more commonly used in Basic-speaking society, either to better fit in or to accommodate others who may have difficulties dealing with their unorthodox sentence structuring.
Vandar is the man. You could say that not every member of Yoda's species speaks that way, it's only a dialect.
Actually Yoda speaks the way he does on purpose so people have to listen (and think) closely, what he says (at least according to the ROTJ-novel, IIRC). Yaddle could simply imitate Yoda (conscious or subconsciously, if she was trained by him).
There's a lot of Yaddle dissing going on here, isn't it? I think the most obvious explanation would be that their culture changed over the course of roughly 4000 years. Besides the fact that the game company wanted to copy Yoda and then messed up the dialogue.
I never got why they decided to kill off Vander off-screen instead of Master Lamar. Probably, Bioware did n ot have the technology to let Vander move around much. Why do all members of Yoda's species end up being Jedi? Yaddle was Yoda's daughter. Do not deny you never thought of it...
If they don't kill him off, then Satele Shan mostly would not have become the Grand Master in TOR since Yoda's race could live very long.