Context :

Aims : We investigate the atmospheric structure and fundamental properties of the red supergiant VY CMa .

Methods : We obtained near-infrared spectro-interferometric observations of VY CMa with spectral resolutions of 35 and 1500 using the AMBER instrument at the VLTI .

Results : The visibility data indicate the presence of molecular layers of water vapor and CO in the extended atmosphere with an asymmetric morphology .
The uniform disk diameter in the water band around 2.0 \mu m is increased by \sim 20 % compared to the near-continuum bandpass at 2.20–2.25 \mu m , and in the CO band at 2.3–2.5 \mu m it is increased by up to \sim 50 % .
The closure phases indicate relatively small deviations from point symmetry close to the photospheric layer , and stronger deviations in the extended H _ { 2 } O and CO layers .
Making use of the high spatial and spectral resolution , a near-continuum bandpass can be isolated from contamination by molecular and dusty layers , and the Rosseland-mean photospheric angular diameter is estimated to 11.3 \pm 0.3 mas based on a PHOENIX atmosphere model .
Together with recent high-precision estimates of the distance and spectro-photometry , this estimate corresponds to a radius of 1420 \pm 120 R _ { \odot } and an effective temperature of 3490 \pm 90 K .

Conclusions : VY CMa exhibits asymmetric , possibly clumpy , atmospheric layers of H _ { 2 } O and CO , which are not co-spatial , within a larger elongated dusty envelope .
Our revised fundamental parameters put VY CMa close to the Hayashi limit of recent evolutionary tracks of initial mass 25 M _ { \odot } with rotation or 32 M _ { \odot } without rotation , shortly before evolving blueward in the HR-diagram .