I understand the legalities of using stock images and vectors for things like logos. But i want to know if it is unprofessional, or negative to use purchased vector graphics in freelancing design work?
It's perfectly ethical. The same as purchasing stock photography for freelance projects. As long as you stay within the purchased license agreement there's nothing to feel bad about.
Now, passing off purchased art as your own original art may be another matter. But, if asked, stating it's a purchased image is generally fine.
Of course you can use purchased graphics in freelance work because it is therefore they are selling in the market. But when it comes to use them on a logo it is not a good idea because a good logo is something has been designed by lots of sketches, research, and design iterations. It is the first thing seen by the audience to know the business behind that logo.
If you are using a vector or any other graphic either it is purchased or not in your logo design then you are not giving your client a personalized service that means you are just trying work smart by incorporating a third-party stuff. You should be fully sincere about the service you are offering to your client although, you can use fonts either it is purchased or not in the logo design but using vector shapes and design in the actual logo that's not fair.
Best of luck.