This may be because it's not really a USB drive, USB drives emit a signal when they are connected to the computer, flash cards do not.
I think I know what the problem is.
The phone emits a signal when connected to usb. However, when you "mount" the card from the android notification panel, it doesn't really emit another signal.
Think of it as a usb card reader on vl-hot.
Since the signal has already been emmited, vl-hot never gets a chance to detect the thing.
Same goes with usb card readers. If you wan to mount them with vl-hot, you must put the card in the reader, and then plug the reader to usb. Unfortunately, this cannot be done with android, because as soon as it detects the USB connection has been interrupted, it restores the sdcard to an unmountable state (from outside the phone).
Bottom line, you can either use hal or mount it manually.