As has been pointed out, the 28% is an outlier. The average poll still has her at around 41%.
If we look at that 41% value, she is still above past vice presidents. Mike Pence eg polled at times around 38% (the list also contains polls from before and after he was VP, so the <30% values are not relevant for this question).
But even if we take the outlier 28%, we can find other outliers that go below that. Dick Cheney regularly polled around 30% (see eg these 29% and 30% polls), and a CBS-New York Times poll found a "favorable" rating of Cheney of just 23% (which at one time even went as low as 13%).