Hi Paul,
Vectors sell very well. Many artists have discovered that vectors move better than photos. I only do photography, so this is second-hand knowledge. However, it seems to be supported by people on the forums at ShutterStock, IStock, etc.
You should not trace a copyrighted image. It will net you bad Karma and is not permissible on some sites. It makes photographers really, really mad.

I've seen a few forum posts where some photographer came across an image that had clearly been traced from their own portfolio. Much unhappiness, there.
Certainly you can use a copyrighted image as inspiration, but what you do should be different enough that the author of the first piece can't tell that you used it for your muse.
You might be able to find non-copyrighted imagery on sites such as FLICKr. You may be allowed to trace these images under some circumstances. Check, because it's not all copyright free. I would ask the artist/photographer for permission either way. My guess is that most amateurs would be flattered and happy to oblige...particularly if you gave them a copy of the vector. Whether that breaks the rules on any given site of the artwork being something you generated wholly by yourself is something you would need to look into on a site by site basis.
I hope this helps...
Good luck.
If you are looking to sign up for any sites, drop me an email (I'm easy to find via Google). I'd be happy to send you a list of site addresses to get you started.