The function \( y = \ln x \) has a vertical asymptote where the function approaches infinity as \( x \) approaches a certain value. For the natural logarithm function, the vertical asymptote occurs at \( x = 0 \).
As \( x \) approaches \( 0 \) from the right (i.e., \( x \to 0^+ \)), \( \ln x \) approaches negative infinity. Therefore, the x-coordinate for the vertical asymptote of \( y = \ln x \) is:
\[ \boxed{0} \]