You should clarify in your help information that the matrix produced has its columns listed in the opposite order of the vander.m function in MATLAB.
Also, providing the determinant is nice, but it would be better to return the log or log10 of the determinant. That way, you can deal with determinants that are very large and very small (it's quite easy for det(A) to underflow or overflow).
Otherwise, nice work, and nicely commented. (You should have a "see also vander" line, though).
Nice use of cumprod. I'm surpised that this function is slower than vander, though.