SELECT 
  id_path 
FROM 
  cscart_categories c 
  JOIN cscart_category_vendor_product_count p USING(category_id) 
WHERE 
  p.company_id = 3

Query time 0.00096

Explain
id select_type table type possible_keys key key_len ref rows Extra
1 SIMPLE p ref PRIMARY PRIMARY 4 const 1 Using index
1 SIMPLE c eq_ref PRIMARY,p_category_id PRIMARY 3 hobilifedt.p.category_id 1

Result

id_path
603/557/734