I think the problem is that spectroscopy at visible and infrared wavelengths simply can't penetrate the thick atmosphere of Venus. Thus the information comes from much higher in the atmosphere and not from the surface.
The Sun doesn't have a surface. However, it does have a reasonably sharply defined photosphere, which is the "surface" from which the light reaching us escapes. The "effective temperature" of the photosphere is then just the temperature that a blackbody of that radius would have to have in order to emit the luminosity of the Sun.
Thus the effective temperature isn't actually the temperature of anything, it is a parameter relating the radius of the Sun to its luminous output. In fact the solar spectrum isn't a blackbody spectrum and its peak combined with Wien's law wouldn't yield the effective temperature.
All this is by way of explaining the difficulties of Venus. The planet will also have a "photosphere" from which light of any particular wavelength will escape. For light at optical and infrared wavelengths this will be well above the surface, and modelling that spectrum will give a temperature way lower than the surface temperature.
In order to get the surface temperature one would need the temperature at this Venusian photosphere, an idea of how high above the surface that was, and some knowledge of the temperature gradient of the atmosphere.
All this can be worked out with an appropriate mixture of physical models and observations at different wavelengths that probe to different depths in the atmosphere.