Well, it's true, if they were able to show episodes all the way from 1986-1998 and ran a decent promotional campaign they'd probably have more than enough viewers to keep it on air. But they wouldn't pay to have the episodes transferred or run promotions UNLESS there was a decent amount of viewers. Obviously, this sort of thing is true anywhere in big business unfortunatly.
Part of the problem is lack of promotion. Prior to launch, the trailer they ran was only shown once every 2 hours which would be quite easy to miss (it also made no mention of the 5PM Thursday showing), and the trailers they ran for individual episodes (the 1998 Valentines episode and the 1997 Special) were shown so infrequently I only saw them a handful of times- and I have The Vault running in the background most of the day.
They should do a "make or break" thing- run a decent promotional campaign (trailers 2 or 3 times an hour- possibly run them on the other Chart Show owned channels too), introduce a showing during the 9-11PM timeslot on one weekday (possibly move the Thursday showing) and see what happens- if it works, it works. If it doesn't, they'll (sadly) have to take them off.
OR at least, shove it into the doldrums- somewhere around 3-4AM. If The Vault is like most other channels (I don't know, I've never actually watched them at this time) they won't be running adverts, therefore viewing figures aren't really as important, and they could at least keep them running for us who enjoy them, where we could record them to watch later. That is if it would be considered cost effective to transfer & edit the episodes just to shove them in such a timeslot- which it might not be.