Same problem here. I did a bit of testing, here a few findings:
1. The problem started with Yosemite.
2. If you have multiple user accounts, the problem is the same for all users.
3. Not only Pages and Keynote are affected: third-party apps such as Swift Publisher 4, Autodesk Graphic (formerly iDraw), Notability, LibreOffice and probably many more apps show the same problem.
4. Not all apps have the problem: Nisus Writer Pro pastes high quality (vector), Scrivener too, Pages '09 (older version, part of iWork), Keynote '09 (older version, part of iWork) work fine. It seems to me that older apps that are not "made for Yosemite" work fine. Best example: Version 3 of Swift Publisher pastes vector, Version 4 (optimised for Yosemite) pastes blurred bitmap… I contacted the developer, but he wasn't able to reproduce the problem on his machine.
5. I use a MacBookPro, mid 2010.
6. For Yosemite, there is a simple workaround: use the open-source Skim.app instead of Preview.app, and you have vector high-quality as before.
7. I updated to El Capitan: the same problem, and even worse: Skim pastes only white boxes (so this alternative is not working anymore).
8. I then did a clean install of El Capitan, with a new user, no apps (apart from pages): the problem was gone, Preview copies high quality vector graphics, and you can paste them into pages. Hooray!
9. Oops, my hooray came too early. After "a while" (I don't know what or if I changed anything), the blurred bitmaps are back again :-/
10. Found new workaround: make your selection in Preview.app, and then click on "Crop". Save this PDF snipped and then drag the file's icon into Keynote, Pages, Swift Publisher or whatever you use. You will get vector high quality graphics…
Anyway, at least I can work now with the crop-drag-drop-workaround. However, this is ridiculously cumbersome of course. I wonder what happened between my clean install of El Capitan (when vector pasting worked) and now (blurred images again). Probably there must be one file or app or some thing that disturbs PDF copy/paste, as Preview would be actually capable of copying the right way. I guess that file sits somewhere not in the user folders, as it affects all users. Any ideas?