Can I get a work permit without the sponsorship letter?

No you cannot. A work permit may be terminated in the following circumstances:

• Expiration of work permit;
• Termination of labor contract;
• The content of the labor contract is not consistent with the work permit granted;
• If the foreign employee is fired by the foreign employer;
• Withdrawal of work permit by authorized state agencies;
• Termination of operation of the company, organization, and partners in Vietnam; and
• The foreigner is sentenced to prison, dies, or is proclaimed missing by the court.