I'm glad to see your post here, eddeee.
You can design your invoice template which includes custom fields, then import it to QuickBooks Online (QBO).
First step is to download the EZ Map Fields sample template or if you have already made a custom design, you can start mapping is using the samples found in the article below.
Everything you need to know about importing custom form styles for invoices can be learn from this article.
Also, there is a specific version of QBO for Vietnam. You can check this out by visiting this link: https://quickbooks.intuit.com/choose-country/.
If there's anything else you need help with, just let me know. I'm always happy to assist. Have a great day ahead.