Count duplicate occurrences in MYSQL table based on value in two columns using having and inner join.
You can reduce the queries to one:

select owner_id, sum(vehicle_type='Car') as cnt_car, sum(vehicle_type='Van') as cnt_van
from vehicles
where vehicle_type in ('Car', 'Van')
group by owner_id
having cnt_car &gt; 1 and cnt_van &gt; 1


Apart from that, the index that would provide the best performance would be a combined index on the columns (owner_id, vehicle_type).