Will owning a business that sells illegal products in Thailand affect my DTV application in Hanoi?

I would say probably not.

The meaning behind all the approvals is yo make sure you are both a net positive influence over Thailand's economy and stability.

Seems you will qualify assuming rest of the financial documents are in order.

Distributing cancer-causing products for a living should not be tolerated. But unfortunately, as long as you have a registered company, payslips, and 500,000 THB, you should be fine. It still depends on the discretion of the officer reviewing your application.

If you apply as business owner ( like myself ) you will need to show your company’s real activities and of course the easiest way for me is to show proof (information and pictures) of 2-3 clients. So if I were you I would provide information of clients/ distributeurs outside of Thailand.

Keep in mind that their job is to make sure that you have enough income/money to spend in Thailand. And embassy officers are not customs or police :-)

I doubt they care as long as you are not doing business in Thailand.