The use of the "Ace of Spades" as a psychological weapon varied in Viet Nam.  There were several variations of this theme.  In its original form this symbol did not have the impact it was intended to have.  Since the deck of cards used by those in the Far East differs from the 'western' deck of cards, the VC and the NVA failed to see the psychological link between death and the 'Ace of Spades'.  Their culture did not include century-old references to this card.

The United States tried to instill this phobia by coupling this symbol with text.  They produced leaflets showing the Ace of Spades and text such as shown below.  Although I do not know what the text states, the style and font suggest that being a VC is not a good idea.  (The text below is actually part of the leaflet above.  The size of the leaflet did not allow scanning both parts into one image. )

Later, the use of this propaganda (psy ops) weapon was modified.  An area of suspected VC activity would be targeted and a variation of this leaflet would be airdropped over the region.  The leaflet would say something like:  "We are coming into this area in a couple days and any VC we find are going to die."   Sure enough... we would then run a 'search and destroy' mission into this area.  The US employed this procedure several times for selected areas.

Later, the US found that it could pick an area and surround it.  The Air Force was called upon to drop the 'Ace of Spades' leaflets throughout the targeted area.  Since the VC knew from experience that US forces were coming in, they would scramble to leave the area.  Our troops would catch them as they tried to run. 

Of course, the usefulness of this tactic wore out once the VC caught on.  But for a little while it worked like a charm and resulted in few US casualties.