Vigenère cipher - sorted order
ADLY is the order those letters appear in the alphabet, so A=1, D=2, L=3, Y=4.

But I've never seen such a substitution in a Vigenere cipher, and it's dumb, since it makes a weak algorithm even weaker. Normally you just identify A with 0 or 1 and then assign increasing numbers to subsequent letters. A=0, B=1, C=2, ... Z=25, so LADY would map to 11,0,3,24 (or 12,1,4,25 if you start with A=1 and end with Z=26=0).