Positive Sequence (ABC) Balanced Delta Load Vector Diagram
The choice of reference angle is arbitrary.  I like to put A-phase voltage on the right axis (labeled 0° in my figure below), but it is just my preference.  I have seen author's who like to put A-phase voltage on the vertical axis and make it the 0° mark. To jonk's point, the author could assign the 0° mark to \$V_{AB}\$.
What matters in phasor diagrams is the relationship between the phasors.  Choosing the arbitrary reference angle merely rotates the entire diagram.
Assuming the voltages are balanced, we know that the A-phase-ground voltage will be within ±30° of the A-B voltage.  That, coupled with the information that this is an A-B-C rotation (the order in which the phasors pass a fixed point on the diagram when rotated counterclockwise) system allow us to located the 3 phase-ground voltage phasors (\$V_A\$,\$V_B\$, and \$V_C\$) as I show below.  From this, you can see that the A-phase current, \$I_A\$, lags the A-phase voltage by 30°.

Note: The \$V_{AB}\$ phasor is \$V_A - V_B\$ which you find by simply negating the \$V_B\$ phasor and adding it &quot;head-to-tail&quot; with the \$V_A\$ phasor.  The resultant being \$V_{AB}\$ which we know is \$\sqrt{3}\$ longer than the phase-ground phasors.