  We show that the liquid-to-crystal quantum phase transition in the
Rokhsar--Kivelson dimer model on the two-dimensional triangular lattice occurs
as a condensation of vortex-like excitations called ``visons''. This conclusion
is drawn from the numerical studies of the vison spectrum in the liquid phase
by using the Green's function Monte Carlo method. We find that visons remain
the lowest excitation throughout the liquid phase and that their gap decreases
continuously to zero at the phase transition. The nature of the crystal phase
and the second order of the phase transition are in agreement with the earlier
prediction of Moessner and Sondhi [Phys. Rev. B 63, 224401 (2001)].

Strongly Correlated Electrons