Vintco as a company are a bit like Allballs. They dont make the parts but assemble kits and for the pistons, as mentioned above, they use Wossner.
ASSO have been around since the 70s and supply pistons to some reasonable engine companies like Pollini Piaggio and rotax and heaps of Italian agricultural machines and here's the but, They also have a plant in India now. That said ,So do Harley, not that they are any recommendation.