This is like some sort of tease! As I look at my posts in my local dev environment, I see only one crisp avatar - the default System user and first post. There really should be a solution for getting crisp avatar photos to work in Vanilla Forums.
Decrease the size of the upload (file size and dimensions), then there would be no need to resize. There are config options.
It also could be a dodgy version of gd library.
grep is your friend.
This did it for me. It allowed me to make retina-friendly thumbnails once the image is uploaded.
$Configuration['Garden']['Thumbnail']['Size'] = 100;