I found the solution to LAN multiplay Age of Empires II: The Conquerors on Vista!
First, rightclick on the age2_x1.exe file. Under properties, select XP compatability. (Not sure whether is really necessary). What IS IMPERATIVE IS THE FOLLOWING: Then, go into the game. When running Vista, MAKE SURE YOUR AGE OF EMPIRES GAME RESOLUTION is on 1024 x 768, and NOT 1280 x 1024. If you run it in 1280 x 1024, the game when playing multiplayer will simply exit, which is very frustrating!!
So put it on the lowest or middle resolution. Then, when the game is going, you can switch from lowest (600 x 800) to 1024 x 768. Further, when running vista, the colour might appear weird and bleachy. The solution is to simply press on the 'return to windows button' (Next to Alt) The return to the game, change the resolution in the game to lowest. Play for 30 secs, the change it to middle resolution (1024 x 768) wait a while and it appears normal.
Hope this helps a few guys, I know several people struggling with it!