We needed another 3rd choice CB regardless of whether Vermaelen stays or not IMO. He's nothing more than a 4th choice CB for me as he'll never seriously challenge Merts or Kos for a place because he's not good enough. We need more than 'just' a back-up to make up the numbers or we'll be fucked if either of those two picks up a long term injury.
It would be £5-10m net after selling Vermaelen which I don't think is unreasonable considering we would (ideally) be investing in a better, longer term player. Even if he wasn't willing to go that high, remember that we bought Mertesacker, Koscielny and even Vermaelen himself for £10m or less. Vidic and Kompany costed £7m! It's hard to believe that we can't find someone at least as good as Vermaelen.
If Kos or Merts get injured we're fucked anyway, Vermaelen has shown that he isn't up to it time and time again. It's always a risk to bring in a player from another league but you have to put faith in your scouting network and judgment of a player.