The later "speedline" vacumatic has a plastic plunger under a much bigger blind cap and it no longer locks down.  To fill, remove the blind cap, immerse the nib in ink and pump the plunger about 10 times, pausing for a second or two at the top and bottom of each stroke.  Remember to finish on a DOWN stroke, lift the pen out of the ink and then release the plunger.  If you release the plunger first, the pen may tend to flood until the excess ink has cleared.  Finally, clean the nib off with a tissue.  Phew! - not the simplest one to fill!