OK I've found the solution !! Problem solved !! Vector is not the culprit..
The problem is : If you have or had a FSX installation and you try to install Tongass into P3D, some of the Tongass files will not be installed, so I had to copy/paste all missing files from FSX folders to P3D folders.
I've found this behavior after reading this on FsAddon forum :
http://fssupport.com/fsaddonforum/index.php?topic=42.0To solve the problem, we need to know what files are missing, so I've found another post on FsAddon :
http://fssupport.com/fsaddonforum/index.php?topic=59.0Also, on this post ;
http://fssupport.com/fsaddonforum/index.php?topic=34.0Holger Sandmann is explaining the process to install Tongass into P3D and he provide a list of all files installed by Tongass (you need to be registered to download the list). With that list on hand, I was able to find all missing files.
Elevation problems were fixed by adding all elevation files missing into \Scenery\World\scenery:
AF2_FSX_TF_CAB5_elevation_adjustment.BGL
AF2_FSX_TF_CBM5_elevation_adjustment.BGL
AF2_FSX_TF_CBW4_elevation_adjustment.BGL
AF2_FSX_TF_PAFE_elevation_adjustment.BGL
AF2_FSX_TF_PAKW_elevation_adjustment.BGL
AF2_FSX_TF_PAPG_elevation_adjustment.BGL
AF2_FSX_TF_PASI_elevation_adjustment.BGL
AF2_FSX_TF_PAWG_elevation_adjustment.BGL
I've also found many other files missing (Effects, Sound, AI ships, AI aircrafts). We also have to add the 40 entries as per Holger instructions.
I think if you don't have or had a FSX installation (FSX or FSX-SE), Tongass will install all the files.
Thanks for your help !! Hope this will help someone else !! Happy flying !