  AIMS. We test the hypothesis of O Vz stars (characterized by having HeII4686
stronger in absorption than other He lines in their blue-violet spectra) being
at a younger evolutionary stage than are normal O-type dwarfs. METHODS. We have
performed a quantitative spectroscopic analysis of a sample of 38 O Vz and 46 O
V stars, identified by the VLT-FLAMES Tarantula Survey in the 30 Doradus region
of the Large Magellanic Cloud (LMC). We obtained the stellar and wind
parameters of both samples using the FASTWIND stellar atmosphere code and the
IACOB-GBAT grid-based automatic tool. In the framework of a differential study,
we compared the physical and evolutionary properties of both samples, regarding
Teff, logg, logQ and logL. We also investigated the predictions of the FASTWIND
code about the O Vz phenomenon. RESULTS. We find a differential distribution of
objects in terms of effective temperature, with O Vz stars dominant at
intermediate values. The O Vz stars in 30 Doradus tend to be younger and less
luminous, and they have weaker winds than the O V stars, but we also find
examples with ages of 2-4 Myr and with luminosities and winds that are similar
to those of normal O dwarfs. Moreover, the O Vz stars do not appear to have
higher gravities than the O V stars. In addition to effective temperature and
wind strength, our FASTWIND predictions indicate how important it is to take
other stellar parameters (gravity and projected rotational velocity) into
account for correctly interpreting the O Vz phenomenon. CONCLUSIONS. In
general, the O Vz stars appear to be on or very close to the ZAMS, but there
are some examples where the Vz classification does not necessarily imply
extreme youth. In particular, the presence of O Vz stars in our sample at more
evolved phases than expected is likely a consequence of modest O-star winds
owing to the low-metallicity environment of the LMC.
