How can I report a trademark infringement to the US Patent and Trademark Office?

Trademark law is a civil matter. So, if you see someone is infringing your trademark rights, your best option is to initially send them a cease and deist letter. If they don't respond or they refuse, your next course of action would usually be to initiate a lawsuit against them, to get them to stop the infringing activity.

Trademark Infringement in Vietnam

The USPTO does not directly deal with this matters. If, however, someone is trying to register a mark that you think is too similar to your trademark, then you can submit an opposition or file a cancelation with the USPTO. Again, these options only apply if the third party is trying to obtain federal trademark protection through an application.

Source: Quora