What exactly does the video rendering quality option do in Sony Vegas Pro?
This is the explanation for the four quality modes, as provided by Vegas tech support long ago:


  Quality: Best
  
  Scaling: bi-cubic/integration
  
  Field Handling: on
  
  Field Rendering: on (setting dependent)
  
  Framerate Resample/IFR: on (switch dependent)





  Quality: Good
  
  Scaling: bi-linear
  
  Field Handling: on
  
  Field Rendering: on (setting dependent)
  
  Framerate Resample/IFR: on (switch dependent)





  Quality: Preview
  
  Scaling: bi-linear
  
  Field Handling: off
  
  Field Rendering: off
  
  Framerate Resample/IFR: always off





  Quality: Draft
  
  Scaling: point sample
  
  Field Handling: off
  
  Field Rendering: off
  
  Framerate Resample/IFR: always off





  Scaling:
  
  These methods come into play when conforming sources that differ from
  the output size. They are also used when panned, cropped or resized in
  track motion.
  
  Bi-Cubic/Integration - Best image resizing algorithm available in
  Vegas. Quality differences will be most noticeable when using very
  large stills or stretching small sources.
  
  Bi-linear - Best compromise between speed and quality. This method
  will produce good results in most cases.
  
  Point Sampling - Fast but produces poor results.





  Field Handling:
  
  This refers to the field conformance stage of Vegas's video engine.
  This includes Interlaced to Progressive conversion, Interlaced to
  interlaced output when scaling, motion or geometric Video FX and
  Transitions are involved. Skipping this stage can sometimes result in
  bad artifacts when high motion interlaced sources are used.





  Field Rendering:
  
  When the output format is interlaced, Vegas will internally render at
  the field rate (twice the frame rate) to achieve smooth motion and FX
  interpolation.





  Frame Rate Resample / IFR (Interlace Flicker Reduction):
  
  Frame Rate Resample:
  
  This kicks in when speed changes are made through Velocity Envelopes
  and/or event stretching. In can also be used when up-converting low
  frame rate sources. This only kicks in if the resample switch is
  turned on and quality is set to good or best.
  
  Interlace Flicker Reduction:
  
  This kicks in if the event switch is turned on and quality is set to
  good or best. See Vegas' documentation for a description of this
  switch.