  We aim to present 70 spectra of 68 new high-redshift type Ia supernovae
(SNeIa) measured at ESO's VLT during the final two years of operation
(2006-2008) of the Supernova Legacy Survey (SNLS). We use the full five year
SNLS VLT spectral set to investigate a possible spectral evolution of SNeIa
populations with redshift and study spectral properties as a function of
lightcurve fit parameters and the mass of the host-galaxy.
  Reduction and extraction are based on both IRAF standard tasks and our own
reduction pipeline. Redshifts are estimated from host-galaxy lines whenever
possible or alternatively from supernova features. We used the
spectrophotometric SNIa model SALT2 combined with a set of galaxy templates
that model the host-galaxy contamination to assess the type Ia nature of the
candidates.
  We identify 68 new SNeIa with redshift ranging from z=0.207 to z=0.98
(<z>=0.62). Each spectrum is presented individually along with its best-fit
SALT2 model. The five year dataset contains 209 spectra corresponding to 192
SNeIa identified at the VLT. We also publish the redshifts of other candidates
(host galaxies or other transients) whose spectra were obtained at the same
time as the spectra of live SNe Ia. Using the full VLT SNeIa sample, we build
composite spectra around maximum light with cuts in color, lightcurve shape
parameter ('stretch'), host-galaxy mass and redshift. We find that high-z SNeIa
are bluer, brighter and have weaker intermediate mass element absorption lines
than their low-z counterparts at a level consistent with what is expected from
selection effects. We also find a flux excess in the range [3000-3400] A for
SNeIa in low mass host-galaxies or with locally blue U-V colors, and suggest
that the UV flux (or local color) may be used in future cosmological studies as
a third standardization parameter in addition to stretch and color.
