If you have a layered photoshop document. save it as a psd, making sure that the layers mode is ticked. When you place this into illustrator, DON'T PLACE. Open directly into illustrator, options, convert layers to objects. Default is to flatten image, but if you need the layers intact do it this way.
If you're transferring a vector image into illustrator, if you paste in as a smart object, you can continue editing it. When you double click on this object in photoshop, it automatically reopens in illustrator, can change it there, and when you save it, this automatically changes in the photoshop.