You should get the full amount of Reputation from all the dungeons that are level 80. You will just need to get the tabards that you want for exalted.
Here is the Argent Crusade Tabard as an example:
For the Record, every mob in heroic mode grants rep towards the Alliance Vanguard/Horde Expedition. Wearing the Wyrmrest Accord, Kirin Tor, The Ebon Blade or Argent Crusade Tabards CONVERTS that rep to the faction whose tabard you're wearing.
List of dungeons that give rep:
- Utgarde Pinnacle (norm)
- Culling for Startholme (norm)
- Occulus (norm)
- Halls of Lightning (norm)
- All Northrend Heroics
Per comment from Wowhead