Let's say I have 5 small projects, which I later would like to combine. I could render an (avchd) clip out of each of them, create a new project, import these avchd clips, and render the final avchd clip. But then I introduce an extra iteration for compression, which will introduce motion artifacts.
This is why I'd prefer combining several projects into a larger one.
Can it be done?
Or maybe the amount of motion artifacts introduced in this other scheme is minimal?

Movie Studio doesn't allow nested VEG files (or .vf files as the case may be). You definitely listed one option - render each out and combine the renders into a "final" project. Otherwise you can open Vegas twice and copy everything from one project to the other. However, there can be complications if you've done anything at the track level.