As far as I concerned, the biggest ID value for vanilla vehicles is 453 (453_veto2_modkit).
If you are using VWE (VanillaWorks Extended) these are the values that are free:
(values from 866-879 range is used by VWE)
880-889 range (890 is also used by VWE)
981-990 (free if VWE_Sce is not installed)
(990-1024 range is also used by VWE)