VMF Save Folder
I want to open my map in Hammer to make some changes you can't do in PTI, but I can't find where it saved it! Does anybody know where it saves the vmf for your maps?
well your files are saved as p2c files which hammer cant open
your last used puzzle is someplace named preview.vmf
but what you should do it export your map
open the developer console with your puzzle loaded and type puzzlemaker_export <name>
what ever you put for name will be the name of the vmf exported to your portal 2 sdk maps folder
Also, when you export it, you should get something with a long string of numbers for a name in /sdk_content/maps/. That's the source file, with slightly different lighting.
How do you import it back into the editor after modifying it?
You can't. You will have to use the 
located in portal 2/bin to publish to the workshop.
By putting all your changes into instances, and editing editoritems.txt (and perhaps your .p2c file) to include those instances.
But you may prefer to just publish your hammer map using p2map_publish.exe instead, like everyone except me does.