I've done some experiments with daily missions and achievements

[AOD_CptCheerios]

So here are my conclusions
For the monthly achievements you dont (get 20k in specific vehicles) you need to only have one of said vehicles in your lineup and use it. All of the points however do county.

I did it with this past weekends achievement where I had 4.3 Germany ground rb and the only vehicle that applied was the bf109 g. I got around 8-9k mission points in the jagdpanzer iv, and 1 kill on the bf109. Got full points every match

As for daily I've only done it so far with unexpected threat, second tier where it's 5 kills rank III AA or higher. I got my first kill with swedish 2.7 l-anti rank III. Then I got two kills with the rank II truck and then two kills with a rank II plane and it counted all 5 kills..

So yeah game has said if you get 1 that's good enough the rest still count. I have yet to try with slow but sure.

[AOD_CptCheerios]

Update: It does not work with slow but sure.

[AOD_Macjagger]

yeah some tasks like 'kill enemies using a heavy tank' won't work unless that heavy is tier 3. But for most of them it does work if you spawn tier 3 1st spawn. I've given up doing that as I eventually forget and miss out. 8) RIP Chaffee at tier 3.

[AOD_fred_brie]

Hope we are talking about the same thing, for the battlepass task:

If it says use the vehicle in the mission: Rank III and above ; Everything counts as long as you have used a rank III vehicle in the mission.

If it says players vehicle: rank III an above, you need to be in a rank 3 vehicle when you accomplish the task.

[AOD_AngryShifty]

I do task gaijin say no u no do task then give 500 ping then I cry