I use Intellicad, it's an Autocad clone. Maybe more power than you need. 
Can export as PDF. I use SNAGIT to screen grab to BMP, JPG, and many 
other raster formats, and then import. If not familiar with Autocad, 
there will be a learning curve, but then if you are ever have to use 
Autocad, that will be a no brainer. Here are some links:
$51 personal edition probably adequate for what you need. I use the 
professional which has 3D, import raster graphics and draw on top. I use 
this a lot, where I scan a DIP pinout drawing with labels, and draw the 
circuit with lines to the pins. There are considerable symbol libraries 
available, everything from electric motors, transistors, to piping 
symbols, or make your own and save them. Then easily scaled, rotated, moved.
http://intellicadms.com/site/

and here's a free Intellicad, donations appreciated:
http://www.bestfreewaredownload.com/freeware/t-free-progecad-smart-intellicad-autocad-clone-freeware-qtwbnnss.html
I tried it, and it didn't seem to have as nice a feel to it, might have 
needed installation tweaking, but should work good for the simple stuff. 
I think it does 3D and the raster import, but hadn't tried it.


Wouter van Ooijen wrote:
> What would you recommend as vector drawing tool for things like block 
> diagrams, signal sequences, simple mechanical pictures, state transition 
> diagrams, etc? These are meant to be imported in a word document, which 
> eventually might become a book. Must be scalable, so no pixel drawing.
>
> I now lean towards, of all things, the eagle circuit editor, because I 
> know how it works an I have a good .eps export tool for it.
>
>   
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