It just depends on how things are implemented. I don't see any issues moving backwards when I try to as I tested a bit last night. I went back a few minutes and things started right back up for me. Cache would be faster since the chunks are local on disk though so that's definitely correct.
Both use memory for reading things as it depends on how you configure it. VFS has buffer-size which is configurable and only is used until a file is closed so it's really only good for sequential reading / playing with direct play since transcoding reads ahead based on your plex setting.