I've never worked with GTA mods but assuming that if you can't actually get LastVehicle when outside of the car (which seems unintuitive to me, given the name) my guess would be to track the last vehicle yourself. When the player is in the car, store LastVehicle as a variable. Then you can refer to it whenever you want.
Update (how to store LastVehicle as a variable):
It's been a while since I did any VB coding and as I said, I've never done any GTA modding so this is just a guess but hopefully I'm not too far off...
Private lastVehicle As Vehicle
If (Not Player.LastVehicle Is Nothing) Then
lastVehicle = Player.LastVehicle
End If