1. Introduction  
2. Sample selection  
2.1. The VANDELS survey  
2.2. The sample  
3. Method  
3.1. Spectroscopic measures  
3.2. Physical parameters  
4. Parent sample versus selected sample  
5. Results  
5.1. Correlations between Ly α EW and physical properties  
5.2. Correlations between Ly α EW and spectral properties  
5.3. Correlations with IS shift  
6. Discussion: the origin of the relation between ISM and Ly α velocity shifts  
7. Summary and conclusions