One of the benefits of UI kits is to create quick prototype. Sometimes, it's hard to explain an idea to your client, that's when UI kit will be really helpful. Here we have Basiliq, a freehand UI kit for photoshop.
Unlike other UI kit, Basiliq using freehand design concept, so from design perspective, you don't have to worry misleading your client, everything is presented in informal hand drawn, sketches and wireframes. It's good to explain your concept and let your client know how each UI element are positioned and its function too. With over 300+ elements, all vector based, you can assemble anything from them. Here we have YouTube, foursquare and facebook:
The problem is that popular prototyping tools produce mockups that don't look so great. That's why we've created over three hundred design elements specifically for prototyping. A hand-crafted set of icons for mockups.Cloud Castle Group
Here we have another Google+ example:
300+ Elements, All Vector Based!
If you are looking for more vectors based UI, you can download vectors here.