Gimp works fine under AmiCygnix & so does Blender natively (though Blender has a high learning curve for beginners). Lightwave 68k perhaps ?
There is also DrawStudio 2.0....which is about as good as Vector graphics apps got on the Amiga. However, even it is very old. PageStream is still being updated, and has some good vector drawing features, but as said above, it is a desktop publishing app with vector graphics thrown in - not as its strong suite.
You CAN get it done on the Amiga as long as your willing to put up with some bugs and frustration dealing with old software.
Or you can just download InkScape for free (opensource) for Linux or Windows (or Mac?) and do it on that without any hassle. Believe me I'm the kind of person that likes to use the Amiga if I can, but Inkscape is much more up to date.
Using the Amiga might be worth a try as long as you don't have a deadline. Oh, and don't use a bitmap image program to design logos. Logos should be scaleable. (well there are some programs that will intelligently convert bitmaps to vectors, but it's better to design the logo as a vector graphic).
I think the main problem if you design a Vector graphic on the Amiga will be exporting it to a commonly readable format for the client or a printing bureau to use. The Amiga used something like D2DR (or similar) and could often export to Encapsulated Postscript as well - but I've often found that the EPS graphics exported by Amiga vector apps are not quite "right" - i.e. they won't load (or won't load properly) into applications by other manufacturers.
You could try setting up a virtual Postscript printer (with Ghostscript?) and print to a Postscript file. I've never really done this but it might work.