This isn't actually a bug but it's a little unintuitive, the VP's behaviour has a bit of randomness to it compared to other politicians. Even with a fully elite liberal cabinet, there is a 1/9 chance that a VP will support making a law more conservative, if that law is already elite liberal (they won't make it less than liberal though). The president signed the bill because if the VP supports it, the president automatically signs the bill unless the VP and president have different alignments.
*edit* actually looking at the code a bit closer, I think I was interpreting the math wrong and it shouldn't be doing this, it seems like there's a >= sign that should just be a > sign so it makes L+ or C+ VPs more slightly more moderate than they're intended to be.