I narrowed it down to problems with gradients and transparencies. I found two workarounds. 1. create an EPS file with each image saved separately (irritating when I have a lot of images that I want to copy and paste into Powerpoint). Use Insert Picture into Powerpoint and it holds as vector after closing and reopening. The madness is having Powerpoint say the file is saved with your beautiful artwork displayed and reopening the file and it's changed. 2. Eliminate transparencies. This worked an image that had a lot of radial gradient fills. However, I have a linear gradient fill on a stroke in Illustrator that bitmaps on closing. I believe there is some type of transparency built into the gradients in Illustrator so it may come down to an inability to convert transparencies into WMF.  None of this excuses making changes without notifying the user.