I'm going to assume that your 'old vanilla SMP' world directory is from before 1.2.*. Please correct me if I'm wrong.
As to what is causing your world to revert back a few months, I have next to no idea. I had a very similar problem back when I was running a Bukkit server. Whilst trying to update the maps to 1.2.* the map would revert itself 2 weeks from the point at which I updated. I drudged up a baseless theory about the Anvil system and how it was possible that it was causing the reverting issue, as there was nothing else I could see that was causing it. I also found that when I would attempt to load a backup it would do the same exact thing, regardless of the age of the backup. That is what led me to believe it was Anvil that was screwing me over, as it was the only variable that was different.
I don't want to give you false information, which is why I stated my theory had been baseless. I have no idea if that was the cause or not. If it was, however, then you could be experiencing something similar with Tekkit's map generation. Seeing as Tekkit obviously has a different system of world generation (be it something as small/large as ore to tree inclusion) then it could be causing some error or other, or some kind of environment that would instigate the reversion of your map file to some time before the initial conversion. Once again this is baseless.
To address the solving of this issue: I think that the most cost effective solution would be to transfer the most important builds over to a newly generated Tekkit world via MCedit. You could theoretically transfer the entire map over (via MCedit), however depending upon your situation and how you wish to run your server this route might not be the best to take. It would deny your players (or yourself if it is just you) access to freshly generated Tekkit materials. It would also take a varying amount of time to port an entire map, based upon the map size. If the map was used a lot and many buildings were made, then it would take quite a long time to port then entire map, and it is possible it will not work unless you do it in spurts, if your computer is not capable that is.
I recommend just porting over the important things into a fresh new world with MCedit. The choice is of course yours however, and whatever choice you do take, I wish you the best of luck.
MOD EDIT: I'd say signing the post is the worst thing about this but so much about this user in particular is bad, including username, avatar, ponies, etc.