After the merger I was disappointed and stopped playing, now I’m back and I don’t understand anything :v
For example, what new function do VIP troops fulfill?
Why is there more recoil on the gun than before? (I haven’t read any patches in months.)
What’s up with the respawn trucks?
Yeah - the tech trees and progression are horribly complicated for people who played a fair bit before the merge - you aren’t alone! 
Respawn trucks are just what they say - a mobile respawn point with attached crew - they’re just a self-propelled infantry squad that gets a few points automatically when people use them - nothing too exciting.
Before I had the German campaign at full capacity, and now they tell me that I no longer have assaults 3 : l
I didn’t see them in the technology tree (maybe I’m blind) are they for sale? (the truck)
The trucks are at tier 2, br 2.
If you mean the others you have seen driving around they are event/premium
what do you mean?
there are prem APC and Free APC (like studebaker)
noup, only changes on sight of some weapons (Specially, usa Thomson family)
Before, premium troops helped you raise the campaign level faster… Now there is no campaign as such… And since they are not useful for raising troops (as I remember, I could be wrong), what use are they now?
Their use is the same as they have always had
they still having 100% experiencie bonus, which directly affect the research points to unlock a tank or a weapon
but the difference now is those premium units are now made BR restricted,
although, you can still take any to high tier, just limited on which can be used at certain lower BR’s
PS, make Boomerang premium BR II please 
