In Harry Potter and the Halfblood Prince, Draco Malfoy uses a Vanishing Cabinet stored in the Room of Requirement to bring in a bunch of Death Eaters to wreak havoc and kill Dumbledore. Before (not) going in for the kill, the two have a bit of a face-off where Draco explains the ins and outs:
‘So tell me, while we wait for your friends… how did you smuggle them in here? It seems to have taken you a long time to work out how to do it.’ […]
‘I had to mend that broken Vanishing Cabinet that no one’s used for years. The one Montague got lost in last year.’
Dumbledore’s sigh was half a groan. He closed his eyes for a moment.
‘That was clever … there is a pair, I take it?’ […]
‘The other’s in Borgin and Burkes,’ said Malfoy, ‘and they make a kind of passage between them. […] I was the only one who realised what it meant – even Borgin didn’t know – I was the one who realised there could be a way into Hogwarts through the Cabinets if I fixed the broken one.’
‘Very good,’ murmured Dumbledore. ‘So the Death Eaters were able to pass from Borgin and Burkes into the school to help you… a clever plan, a very clever plan… and, as you say, right under my nose…”
Harry Potter and the Halfblood Prince, ch. 27 ‘The Lightning-Struck Tower’
Going by this, Dumbledore clearly didn’t know that the Vanishing Cabinet was part of a pair. I’ve just realised that there’s a bit of a snag with this, though.
We know from Order of the Phoenix that the Vanishing Cabinet in question was not always stored in the Room of Requirement. The first time we hear about it, when Fred and George push Montague into it, it is located somewhere where students roam freely:
‘Malfoy just docked us all about fifty points,’ said Harry furiously, as they watched several more stones fly upwards from the Gryffindor hour-glass.
‘Yeah, Montague tried to do us during break,’ said George.
‘What do you mean, “tried”?’ said Ron quickly.
‘He never managed to get all the words out,’ said Fred, ‘due to the fact that we forced him head-first into that Vanishing Cabinet on the first floor.’
Wherever exactly the Vanishing Cabinet was at that time, it was on the first floor, in a location where members of the Inquisitorial Squad and students (well, Weasley twins at least) both found their way during breaks between classes during the day. So presumably in a corridor or hallway somewhere, or perhaps a seldom-used classroom.
Now Draco does say above that it was broken and “no one’s used [it] for years”—but presumably it must have been functional at some point earlier on. As TGnat points out in the comments below, Nearly Headless Nick convinced Peeves to knock over and severely damage a Vanishing Cabinet on the first floor (or rather: right above Filch’s office, which is listed on the Wiki—without sources—as being on the ground floor) in Chamber of Secrets (ch. 8 ‘The Deathday Party’). If this is indeed the same one, it was functional only about four and a half years previously.
Leaving aside the question of why Dumbledore would leave a non-functional (and possibly dangerous) Vanishing Cabinet standing in a public place like that, I can’t imagine how he could not know what it actually was.
The only likely way for that to come about would be if it had been broken all the time Dumbledore had known it be there; that is, at least since before his time as a teacher (so at the very least about 60 years) and perhaps even as a student (so at least 90 years). This seems unlikely. Who leaves a broken Vanishing Cabinet in a public corridor for 90 years? (It certainly couldn’t be the one from Chamber of Secrets in that case.)
But if the Cabinet had been in working condition while Dumbledore was there as a teacher or headmaster, surely someone would have used it at some point and realised the passage it forms with the one in Borgin and Burkes, and Dumbledore would not have been so surprised to hear of this passage in the tower.