hi,
make a trigger ... triggered by west. Write under condition:
count thislist == 1 && vehicle player in thislist
This will trigger your desired condition immediately. And, it will trigger a player even if he is sitting in a vehicle or not.
I would find it better to delay the trigger a bit:
Set the trigger timer to "Timeout", write into all 3 timing boxes 10,10,10.
This version will trigger only if your condition stays for at least 10sec, but not if another westunit is running into the trigger during the 10sec.
If you wanna use the trigger for MP, i think it will work too. But it is always better to declare the unit that shall meet the condition exactly, by giving it a name like (for example): soldier1
The trigger condition then is:
count thislist == 1 && vehicle soldier1 in thislist
Hope, it will help you...