new Hydra[Amount of hydra + 1 in here];

Hydra[0] = CreateVehicle( ----- ); // create all the hydras with this variable

OnPlayerEnterVehicle
if( PlayerInfo[playerid][pVip] < 1 ) // checks if the player isnt a vip, change the variable to yours
{
    new Float:pos[3];
    GetPlayerPos(playerid, pos[0], pos[1], pos[2]); // gets the player position 
    SetPlayerPos(playerid, pos[0], pos[1], pos[2]); // sets the player position so the player cant enter the vehicle
    SendClientMessage(playerid, COLOR, "You are no vip and cannot enter this vehicle! ");
}