How to vet an overseas travel agency

I am an American planning a trip to Vietnam, and I was planning to book through a travel agency located in Vietnam. In researching the travel agency, I found a lot of reason to feel good about the company:

1. They have good reviews on TripAdvisor, including a "Certificate of Excellence."
2. They have a professional looking website.
3. They have been responsive and professional in email communications.
4. They accept credit card payments through OnePay, which seems like a secure payment method.

As I was getting ready to pull the trigger and book with this agency, a few things made me nervous about the company:

1. I noticed one negative review of the company, in which a traveler felt they may have been scammed by the company.
2. One of the modes of payment listed was bank transfer to the individual bank account of a manager at the company (as opposed to a bank transfer to a company bank account). It struck me as unusual to ask for payments to an employee account instead of a company account.

This has really motivated me to take more steps to vet the travel agency before booking -- I would really hate to travel all the way to Vietnam only to learn that my trip had somehow been switched or canceled.

In researching how to vet companies, a lot of the advice I've found seems difficult to apply here:

1. I don't know of any authority websites (like the Better Business Bureau in the US/Canada or Companies House in the UK) that I could use to look up Vietnamese companies.
2. I don't know how to check the credit report of an overseas company (or if that's even possible in this case).
3. I can't physically visit the company.

How should I proceed in vetting this particular agency beyond the steps I've already taken? More generally, how should one vet an overseas travel agency?