  The shape equation and linking conditions for a vesicle with two-phase
domains are derived. We refine the conjecture on the general neck condition for
the limit shape of a budding vesicle proposed by J\"{u}licher and Lipowsky
[Phys. Rev. Lett. \textbf{70}, 2964 (1993); Phys. Rev. E \textbf{53}, 2670
(1996)], and then we use the shape equation and linking conditions to prove
that this conjecture holds not only for axisymmetric budding vesicles, but also
for asymmetric ones. Our study reveals that the mean curvature at any point on
the membrane segments adjacent to the neck satisfies the general neck condition
for the limit shape of a budding vesicle when the length scale of the membrane
segments is much larger than the characteristic size of the neck but still much
smaller than the characteristic size of the vesicle.
