In its current configuration, the VLT Interferometer (VLTI) combines the light collected by two telescopes and directs it towards the commissioning instrument called VINCI. In an interferometer, the optical path ranging from a telescope to the point where beams are combined is referred as an arm of the interferometer. This arm contains a large number of optics that have to be aligned at installation time and kept aligned during the period of use of the interferometer. The method used to perform the initial alignment is reported in a separate article. This paper is focussed on the methods used to assess the stability of the image alignment of each interferometer arm. Collected data sets are presented and interpreted.