hmmm I might need to fresh up my finnish a bit then. However I'd still translate it as "only" in your given example. ("..., only don't stay there for long.") In any case I admit it would be a "but"ish sort of only.
Maybe my english translation would be incorrect, too? Maybe I'd translate it as "only" only because the German translation is "nur", which would be "only" again in English... Language is weird, especially the english one. Feel free to hit me on the head with the Nex (or Xon, as the case may be) next time you meet me online.
Edit: "vaan" also has other meanings that don't translate to anything like "but" as well, like in "älä vaan...". Which translates to "don't you (dare to)...".
Anyway. You are most probably right and google too (grmbl) but it felt good to slap google once in a while.