Sorry for the Delay
Sorry it took so long to get back, but I was trying to fix my Vanilla that I bought used. It had rebound issues according to the seller on Ebay. Hence when I bought the fork it was based purely on the great price he was selling it at. When I also saw a like new dampening cartridge on Ebay I scooped it up, for $20. I thought I was not going to need to use it, thats why I made you the offer, but last night I finally got to the bottom of the issues my Ebay fork had. I'll summmarize for you because I think some of these might apply for your situation.
1) I bought the fork used on Ebay. The seller was up front about the rebound issues, but claimed he did not know why.
2) When I got the fork, it had basically no oil inside. I cleaned it, reassembled it, but the top cap on the dampening cartridge side was really hard to screw in. I noticed one of the extra springs he had sent me was 200mm, the other two were 240mm. When I compared it to Vanilla I had bought new 2 years ago, it does take the shorter spring on the dampening side rod.
3) Next I had to find a way to get the spring off the rod and put the correct spring on. I asked on this message board and my LBS, but no answers. I was planning to resell the whole fork on Ebay.
4) Finally, last night I was doing a search on Vanilla Lockout Levers, and someone explained you can disassemble the cartridge side by the screw on the bottom. Success!
5) I ended up using the dampening rod/cartridge I bought on Ebay because it felt smoother than the one that had the too long spring. Who knows how long my "honest" seller had been using the fork with the 240mm spring.
Now I have a sweet Vanilla that cost me 125 for the fork + 20 for a like new dampening rod/cartridge + lots of frustration = $145, but I know the Vanilla internal workings a whole lot better!
I have become much more knowledgeable on the internal workings of the Vanilla. I was really able to see how the hydraulic lockout works.
As for your situation, how well do you know the seller? Mine was not to up front about what had happened to his fork. I find difficult to believe you change the springs on the fork, but don't remember doing it. As for the cartridge, contact Fox first and find out how much it would cost you. I know there replacement parts can be pricey. I once looked into getting a new steerer tube, but you have to get a whole new upper assembly for $150, so I can see fox charging close to $100 or more for a new cartridge, but this is purely a guess.
As someone else suggested, take it to your LBS and have them give it a look.
Good Luck!