[ReVo_]

new bool:iVUsed[MAX_VEHICLES] = {false, ...};

OnPlayerStateChange

if (newstate == PLAYER_STATE_DRIVER)
iVUsed[GetPlayerVehicleID(playerid)] = true;

In the command:

for (new i = 0; i < MAX_VEHICLES; i++)
if (iVUsed[i])
SetVehicleToRespawn(i), iVUsed[i] = false;