The in-plane structure functions in the liquid-vapor interfaces of two bismuth-gallium alloys (0.18 at. % Bi and 0.07 at. % Bi) have been studied by grazing incidence x-ray diffraction. The higher concentration alloy exhibits, at 25, 43, and 77 degrees C, segregation of Bi into nearly complete monolayers atop the bulk alloy; at each temperature the Ri monolayer has the structure of a two-dimensional supercooled liquid. The intensity distribution of the diffuse x-ray scattering at q(z)=1.0 and 1.5 Angstrom(-1) is used to calculate the surface tension of the alloy at 77 degrees C; the value obtained is 570+/-30 dyn/cm. This value, which is very different from the value obtained by extrapolating the surface tension of liquid Bi to 77 degrees C, is related to the structure of the Liquid-vapor interface. We have compared experimental grazing incidence x-ray diffraction studies on the liquid-vapor interface of the 0.07% alloy with integral-equation theoretical calculations on a two-dimensional hard-disk fluid mixture. The results suggest that the segregated partial monolayer may consist of dimers and trimers in equilibrium with a small amount of two-dimensional Liquid bismuth.