First the mount -t vfat .. is the good line.
the fsck will not work on vfat
Have you tried reading it on another OS like windows?
I'm wondering if you did not unplug the usb without unmounting it, or if you didn't format it with something else.
what gives this:
modprobe vfat if it is not in lsmod
mount /dev/sda1 /mnt/usb
oops, from your first post:
fsck.ext2: No such device or address while trying to open /dev/sda1
this would indicate that you have a usb problem.