This is because you are applying EquiJoin. You need LeftJoin
from user in ObjDT_Usuario.AsEnumerable()
join car in ObjDT_Vehiculos.AsEnumerable()
on user.Field<string>("ID") equals car.Field<string>("ID") into userxs
from u in userxs.DefaultIfEmpty(
orderby u .Field<string>(campo)
select new UserReport
{
Name = user.Field<string>("Name"),
Car = u.Field<string>("Car")
};
Please ignore Typo mistakes. I am not on my laptop.I hope you got an idea