Teaching In Vietnam Less Than 3 Months

Hi and welcome to the forum,

You can work (teach) in Vietnam as an English teacher for less than three months without a WP, provided you're on a Business visa sponsored by your employer.
In other words your employer is expected to give you an invitation letter based on which your B-visa should be issued.
That's the legal side of it.

It's illegal to work in Vietnam on a tourist visa.

A degree is one of the minimum requirements for a successful Work Permit application, in case of an English teacher.
That said, I'd suggest not to give up your hopes completely.
Bring all your documents (originals) with you and give it a try while in Hanoi.
You just never know!

However, I am a writer/journalist with 5 years experience.

- do you have any diploma/certificate in Journalism or related subjects? If yes, please bring the original and a notarized copy with you.

- try to summarize your 5 yrs worth of experience as a journalist/writer in a well-presented document and get it endorsed by a competent authority/organization in the same field.
For example, in all those five years, if you were working for a news agency, get a well-written Experience Certificate or Statement of Employment (highlighting your published literary works in English) from them.

- If possible, get a copy of that experience certificate notarized as well.

There are many IELTS training centers in Hanoi. Your experience as a writer/journalist could be relevant there as a Writing instructor. 
Also, that way it might be easier for your employer to persuade the labour dept. to issue a Work Permit (WP) for you.
You get a Work Visa only after you have your WP.

Hope this info helps!
Welcome to Vietnam.