'Bulk' could mean anything from a 1 tonne IATA pallet to chartering a weekly 747, but having air-freighted 12 tonnes from Australia to UK a couple of months ago I would offer the following advice:
Use a good shipping agent where you can talk to real people, ideally with a physical presence at the origin and destination. There are many pitfalls to trap you otherwise.
Do not ship by air during the height of a worldwide pandemic. The direct cost went up by over 5x and there were significant indirect costs associated.
Design your packaging to fit IATA pallets and containers. Ours were the absolute largest in all dimensions to fit a non-freight aircraft. This saves significantly on cost.
Try to ensure a direct flight. Our wooden crates ended up outside on the pan in Hong Kong during a couple of storms resulting in damage to the crates and contents. This was unavoidable due to CV-19. You should not have a problem with Vietnam > US.
Be very clear with the agent wrt the cost / schedule compromise. There can be savings to be made by longer land journeys at either end of the flight. A totally made-up example - it may be cheaper to ship from Vietnam to San Francisco then truck to Los Angeles rather than ship direct to LAX. It all depends on route availabilty.
Likewise, make sure the shipper applies the correct priority. If you need to guarantee a certain flight you will pay through the nose but if you can accept any flight over, say, a 3 day period then you will save.
Make sure that the shipper understands whether this is a 1-off or regular shipment. There are some good deals to be had if you have regular, scheduled requirements.
Don't get my started on Customs clearance. Let your agent do it but make sure the correct commodity codes are used.
Outside of a pandemic, I am always surprised that long-haul shipping is relatively good value compared to the disadvantages of the cheaper alternatives. That is in a professional context. I would hate to have to pay for it personally!