The history of plates of Vigili Urbani (local police with control of traffic) of San Marino is the same as that of plates of Gendarmeria: at the beginning they were like civil plates but with a little red print "VIGILI URBANI" on the bottom, as you can see in the first two pictures (the second is taken from a motorcycle). Later a new plate was introduced, with the coat of the arms of San Marino printed with polychromatic techniques on the left, the letters VU in red and then 3 digits with leading zeros if needed (third picture).
The Corps of Vigili Urbani was incorporated in Polizia in 1983, like Gendarmeria, and started using its same plates.
Many thanks to Giancarlo Raposo for the first picture. The second picture is taken from the issue of October 1969 of the ALPCA Newsletter and I must thank Joe Sallmen who sent it to me. The third picture is taken from the archives of ALPCA.