  The VLTI Spectro Imager project aims to perform imaging with a temporal
resolution of 1 night and with a maximum angular resolution of 1
milliarcsecond, making best use of the Very Large Telescope Interferometer
capabilities. To fulfill the scientific goals (see Garcia et. al.), the system
requirements are: a) combining 4 to 6 beams; b) working in spectral bands J, H
and K; c) spectral resolution from R= 100 to 12000; and d) internal fringe
tracking on-axis, or off-axis when associated to the PRIMA dual-beam facility.
The concept of VSI consists on 6 sub-systems: a common path distributing the
light between the fringe tracker and the scientific instrument, the fringe
tracker ensuring the co-phasing of the array, the scientific instrument
delivering the interferometric observables and a calibration tool providing
sources for internal alignment and interferometric calibrations. The two
remaining sub-systems are the control system and the observation support
software dedicated to the reduction of the interferometric data. This paper
presents the global concept of VSI science path including the common path, the
scientific instrument and the calibration tool. The scientific combination
using a set of integrated optics multi-way beam combiners to provide
high-stability visibility and closure phase measurements are also described.
Finally we will address the performance budget of the global VSI instrument.
The fringe tracker and scientific spectrograph will be shortly described.
