I understand that all the multiplayer map pack will be included in the multiplayer part of Halo 3: ODST. But will it have separate achievements from Halo 3 or not? 
 
For example: I have Halo 3, and i haven't got any of the map packs except the free one. So i have been unable to get any of the Mythic or Legendary exclusive achievements. Like the 
Vidmaster Challenge: Brainpan achievement which is one of the seven required achievements in Halo 3 and ODST for unlocking the Recon armor.
 
Will i be able to get that achievement using the multiplayer part in OSDT, or do i have to transfer over the map packs to Halo 3 somehow using the disc? Or worst scenario, do i have to buy the mappack separatly from ODST?
 
Please use the reply feature when replying to me, thanks.

The achievements on the second disk link to the Halo 3 achievement list, so you'll be able to earn those Legendary/Mythic achievements that you don't have. The entire Halo 3 multiplayer mode is included with the maps on the second disk, so you won't have to download anything extra.

The 2nd disc of ODST is the entire halo 3 online component and the every map pack released so far on xbox live, including the unreleased mythic part 2 map pack.
 
You can either load that disc *which should load the maps onto your system* and play the original halo 3, or play the multiplayer component on that disc and get said achievements, but some of the vidmaster achievements you need are obtainable only on coop halo 3 single player *I.E the vidmaster i believe*

" The achievements on the second disk link to the Halo 3 achievement list, so you'll be able to earn those Legendary/Mythic achievements that you don't have. The entire Halo 3 multiplayer mode is included with the maps on the second disk, so you won't have to download anything extra. "

"but some of the vidmaster achievements you need are obtainable only on coop halo 3 single player *I.E the vidmaster i believe* "