Ship-vanish is a rather dangerous SEXP to use carelessly as certain things ignore it. If you want to fake it simply use the set-x(y,z)-position SEXPs to move the ship 200,000m away and stealth it if you can.
You may be able to ship-create a ship that you vanished earlier but you could possibly get some very strange bugs out of that so experiment first before relying on it.
I would already be doing this if my plan wasn't to move a large amount of ships. (We're talking everyone but the player.)
Also warping might work, but as I said, I'm moving everything, so this would have to include ships not in system yet as the effect I'm having is supposed to be slightly random.
[edit]
Okay I'll explain what I'm trying to do step by step, even though I really didn't want to.
Anyway, I'm trying to make it look like tho player is being transported to a nebula for a short few seconds and back again, quickly and painlessly.
So, turning on the nebula is no problem at all. The problem is making the rest of the ships disappear. I tried to rectify this moving the player 70 odd km away, while saving the coordinates and them moving them back after.
However, they still appear on sensors and are still targeted by the player if you have them targeted when you teleport.
So I tried ship-invisible, they're still targeted by the player, but are now gone from the radar. I realised that I could kinda stop this by starting an EMP storm, which knocks off what you have targetted.
Now, although not without flaws, this is the best I colud come up with, until I realised I was now flying into ships. It took me a while to narrow it down to the ship-invisible, ship-visible combination.
So if anyone has any fix or suggestions it would be appreciated. My mission revolves around this effect so it's pretty critical. Otherwise it's back to the story drawing board