My Vantage Vue is three years old and just recently experienced the same problem with wind speed as several have described here. I thought about this for several days this past week and decided to disassemble the outer case and take a closer look at the sensor and board.
I removed that pyramid-wedge shaped piece of plastic over the board the houses the anemometer sensor and the temperature sensor below. I pulled that up and out and the cable appeared to be intact and the entire board coated and no cracks or breaks.
I then took a magnet and ran is past the square magnet that is mounted at the end of the anemometer shaft, which theoretically rotates when the cups go around and in turn activates that reed or switch to send the signal to the ISS. My thinking on doing that was that possibly that little square magnet had lost some of it's magnetic field (if that's the way to describe that).
That seemed to improve the speed sensor for a few hours, but then it stopped again.
So thinking that just maybe the magnet was weak or the reed was, I noticed that where you put this board back in place, that there was a bit of sponge material around the temperature sensor end of that board and when it was put back in its' original position there was some room (space) above the anemometer end of that board, that one could raise that board up a little, pack some material (sponge, or other insulating material) around the temperature end, keep it vertical and replace the protective (Wedge shaped cover).
My anemometer is now operating as it should and I believe that the board being closer to that magnet on the end of the cups shaft is more readily activating that reed switch.
Might not work for everyone, but it did solve my problem and saved spending a few bucks at this time. Incidently, I gave my rain gauge the same treatment awhile back, wherein it appears that magnet on it needed a little recharge as I found the pcb board for it was still sealed well and did not appear to have been injured from moisture or age.