Well, a few things of note on this achievement......it really tracks player kills.
Thanks to the low player count of War Thunder in General and Sim Battles especially this might pose a problem, as sometimes when there is a lack of players the rest of the places is filled up by "player"-bots, now these count as player kills for all ingame challenges, but not for this achievement.
So to get this, you will actually have to kill 100 vehicles actually driven by players.
If you don't get it by the time you have 100 kills in Sim Battles, this doesn't mean that this achievement is bugged, it just means that you haven't killed 100 players yet.
I got this at the time when I had 234 kills in Sim Battles for example.
In case your wondering, that is due to the game mode for sim being Enduring Confrontation, where there are a lot of AI aircraft both as support or targets, so racking up more than a few AI kills per match, even more than Player kills is normal.
Also the AI isn't that hard to shoot down, they are epically bad at air combat, so they often get farmed. And often, in case of bombers or attackers, they are the mission goal.