[Threshold]

Ah, but ask yourself this, where are you actually formatting 'string2'? If there is no value for string2, you are not adding anything to 'string'. In fact you're just doing:
string =
addnothingtostring
string =
addnothingtostring
//...
Send(string);

pawn Код:

new string[128], string2[40];
            format(string, sizeof(string), "{FF0000}Vehicle Deleted. | ");
            format(string2, sizeof(string2), "Vehicle Server ID: {FF0000}%d | ", vehicleid);
            strcat(string2, string);
            format(string2,sizeof(string2), "Vehicle MySQL ID: {FF0000}%d | ", VehicleSQLID[vehicleid]);
            strcat(string2, string);
            format(string2, sizeof(string2), "Next Free SQLID: {FF0000}%d", GetFreeMySQLSlot("vehicles"));
            strcat(string2, string);
            SendClientMessage(playerid, COLOUR_WHITE, string);

Should work :P

EDIT: I knew I shouldn't of walked away before posting, 2 others got in before me. I did find it a little weird as to why you were using strcat for this, and there was nothing separating the texts... so I would assume that you're trying to do what SuperViper was suggesting. In that case:

pawn Код:

new string[40];
SendClientMessage(playerid, 0xFFFF00FF, "Vehicle Deleted.");
format(string, sizeof(string), "Vehicle Server ID: {FF0000}%d", vehicleid);
SendClientMessage(playerid, COLOUR_WHITE, string);
format(string, sizeof(string), "Vehicle MySQL ID: {FF0000}%d", VehicleSQLID[vehicleid]);
SendClientMessage(playerid, COLOUR_WHITE, string);
format(string, sizeof(string), "Next Free SQL ID: {FF0000}%d", GetFreeMySQLSlot("vehicles"));
SendClientMessage(playerid, COLOUR_WHITE, string);