You have to get the 10 kills in the metro and 2 assist ribbons in the same round. If not, the F2000 won't unlock.
For the Gol assignment you have to get the sniper rifle ribbon and the 5 kills while on the towers in the same round.
Kind of sucks DICE doesn't exactly spell out how to get the assignments.
AS val is easy, but for some reason, some helicopters do not count torwards the 10 minutes. I think it had to do with one of the transport choppers.
To get the DAO-12, you have to raise or lower the barricade in caspian border and get the shotgun ribbon in the same round.
M60 is also pretty easy. Just get a total of 5 vehicle destroys in oman and an anti vehicle ribbon. I'm not sure if you need to get the ribbon first or not but for the vehicle destroys, they do not have to be in 1 round.