Some (not any) codecs need to have resolutions used which must be divisible by 8. So does WMV. Vegas automatically corrects your input to be divisible by 8.
"Is there a list available for thelimitations of each codec and rendering option?"
I've never seen one and it'd be pretty big and complex.
Another reason could be that some things a codec will accept at your end the codec at the other end may not decode the same due to different implementations. Pixel Aspect Ratio and WMV is a bit hit and miss. Generally I stick to 1.00 and letterbox 16:9.
WMV prefers the image dimensions to be divisible by 8. DivX prefers it to be divisible by 16. Here's a chart for acceptable image dimensions for 4, 8 and 16 in both 4:3 and 16:9.