Then he was the son of a Brahmin dad and Nishada mom (I doubt Nishadas are Shudra) but even if they are Vyas wouldn't be Shudra, just because of mom's caste
Anyway Vidur was legally the son of Vichitraveer, born due to the Niyog of his Dasi, so if his mom was a Kshatriya too, he would have been a Kshatriya.
For him to be a Kshatta his mom should have been a Shudra