It may seem that there is no reasonable explanation for a vip after the tank gets hydro tested.But, there is ..Hydro facilities do not always get all the water out of a tank after testing, and use reuse water very often that has all kinds of dirt in it. The hydro facility we use does not recycle its water and their water bill is high at their location and are now starting to reuse the water..Steel tanks very often develop a bit of rust that can be easily removed but a short period of time an the tumbler machine or a quick brushing. For the $15. the vip is performed, o rings replaced,a bit of cristolube on threads to prevent binding and if the exterior of valve is cleaned up, new sticker placed on it and the tank is now filled. So if the fill normally costs $7. the vip and eddy testing on a $1,000. machine that is used on AL tanks actually now only cost $8. not the $15. stated here. If it is a tank for nitrox use that does not require a hydro and just a vip then in addition to all the steps already listed an inspection with a uv light on the interior to check for any contamination that could be an issue for partial pressure fills.