Now there is in Vegas the "project properties", seems simple and clear... or not?
Let's say i bring some HD material which has to burned to a regular mpeg2-dvd.
Do i set the "properties" according to my input material, in this case the HD-format? Or do set the "properties" according to my end result, in this case the dvd-sized format?

The usual recommendation is that you set your project properties to match your source material.
I used to know what the reasoning was behind this statement but have forgotten it over the years :-)