  Vacuum energies are computed in light-cone field theories to obtain effective
potentials which determine vacuum condensate. Quantization surfaces
interpolating between the light-like surface and the usual spatial one are
useful to define the vacuum energies unambiguously. The Gross-Neveu, SU(N)
Thirring, and O(N) vector models are worked out in the large $N$ limit. The
vacuum energies are found to be independent of the interpolating angle to
define the quantization surface. Renormalization of effective potential is
explicitly performed. As an example of the case with nonconstant order
parameter, two-dimensional QCD is also studied. Vacuum energies are explicitly
obtained in the large $N$ limit which give the gap equation as the stationary
point.
