In 1970, Letterman employees totaled approximately 1100 officers and 750 civilians. Patients were also shuttled from the Travis Air Base in Solana County to the Letterman Hospital using the Crissy US Army Airfield. After World War II a paved runway replaced the grass landing field and the Sixth Army Flight Detachment used Crissy Field for light utility and passenger planes, and helicopter operations. During the Vietnam war the Army used Crissy Field for liaison flights and MedEvac flights to transport wounded Vietnam soldiers 40 miles from Travis Air Force Base to the Presidio's Letterman Army Hospital, a trip by ambulance on surface roads would take too long and possibly be delayed by traffic into San Francisco. At the end of the Vietnam war in 1974 the Army closed Crissy Field to airplanes, though helicopter operations continued for several years. From Wikipedia.
Could the Letterman Hospital or Crissy Army Airfield supply any connection to our killer, either as employee or patient? Had our killer served in Vietnam during his 'quiet' periods? Was the Travis Air Force Base relevant? It is located near Fairfield, California, an area that supplied two pieces of Zodiac correspondence on December 7th and December 16th of 1969. The December 16th letter contained the text "this state is in trouble..I will go for the Goverment life", suggesting more of a connection with the Unabomber Theodore Kaczynski, but in view of American intervention in Vietnam controversial with many, a threat towards officialdom would carry some weight with disgruntled servicemen and the like. The Wing Walker boots responsible for the footprints at Lake Berryessa were issued to military personnel, but were also sold as army surplus from the Travis Air Force Base at Fairfield.
Rosa Vasquez (20) of 834 Bush Street was discovered close to the Arguello Boulevard entrance of the Golden Gate Park on May 29th 1973, the day after Memorial Day. She had been strangled and thrown from the nearby road. This area was just one mile from the corner of the Washington and Cherry intersection. The murder of Rosa Vasquez has not been generally considered to be Zodiac related, having been grouped into other similar murders - notably Angela Thomas (16), Yvonne Quilantang (15) and Nancy Patricia Gidley (24). See here for more info.
Rosa Vasquez worked as a Keypunch Operator. A keypunch is a device for precisely punching holes into stiff paper cards at specific locations as determined by keys struck by a human operator. Now that rings a bell with the Zodiac, in particular the Pines and 13-Hole postcards, the former of which is supposedly referencing the disappearance of Donna Lass. Could the killer have worked alongside both young women, bearing in mind we have two possible murders inside of three years with respect to employees of the Letterman Hospital - what are the odds? As it stands, this is still a weak association - and 'clutching at straws' comes to mind