There are 4 Vidmasters in ODST, the easiest is to collect the 3 skulls on the 3 new mythic maps. The 2nd easiest is classic which is to get through an entire level without throwing a grenade or shooting. The best level to do this on during the campaign is on the level with the wraths, choppers, ghosts, etc. This is the video I used to help me with that one: http://www.youtube.com/watch?v=FbBw7g8r_Fw
The next 2 are the hardest because they require 4 players and it will take you hours. Complete the last level of the campaign with 4 players on legendary with the iron skull on, and no one can use a scorpion or a warthog. Luckily they provide 4 mongooses and rockets with 1000 rockets at the beginning of the highway section, just go down the first tunnel that leads below the ground and you'll find them at the bottom with the rockets on the ground behind them.
The final vidmaster is to get past the 4th set of firefight with 4 players on heroic. This has much harder then it sounds, even if it's on heroic and not legendary. Both these final vidmasters will require a few hours each so make sure that you and you're teammates have a few hours free to dedicate to these.
