Yes, Vaishali left the serial. But then instead of killing Praveena, they should have brought some other actress to play the role of Praveena. Viewers are used to Vaishali as Praveena. Therefore, it would not have been easy for them to accept somebody else in her place. But it would have been far better than killing Praveena. If that new actress were good, then viewers would start liking her also.
One argument given in favour of killing Praveena is that BBB shows real life incidents. I do not know what was age of Praveena in BBB when she died. I agree that in real life it happens that people at this age die. But in real life it also happens that people at this age remain alive.
Therefore, if they did not kill Praveena, then also it would not be unrealistic.