  {Aims.} We introduce our imaging survey of possible young massive globular
clusters in M31 performed with the Wide Field and Planetary Camera 2 (WFPC2) on
the Hubble Space Telescope (HST). We present here details of the data reduction
pipeline that is being applied to all the survey data and describe its
application to the brightest among our targets, van den Bergh 0 (VdB0), taken
as a test case. {Methods.} The reddening, the age and the metallicity of the
cluster are estimated by comparison of the observed Color Magnitude Diagram
(CMD) with theoretical isochrones. {Results.} Under the most conservative
assumptions the stellar mass of VdB0 is M > 2.4 x 10^4 M_sun, but our best
estimates lie in the range ~ 4-9 x 10^4 M_sun. The CMD of VdB0 is best
reproduced by models having solar metallicity and age = 25 Myr. Ages smaller
than = 12 Myr and larger than = 60 Myr are clearly ruled out by the available
data. The cluster has a remarkable number of Red Super Giants (> 18) and a CMD
very similar to Large Magellanic Cloud clusters usually classified as young
globulars such as NGC 1850, for example. {Conclusions.} VdB0 is significantly
brighter (>~ 1 mag) than Galactic open clusters of similar age. Its present-day
mass and half-light radius (r_h=7.4 pc) are more typical of faint globular
clusters than of open clusters. However, given its position within the disk of
M31 it is expected to be destroyed by dynamical effects, in particular by
encounters with giant molecular clouds, within the next ~ 4 Gyr.
