Working as an English teacher for less than 3 months in Vietnam?

It is unrealistic to work as an ESL instructor for three months or less, particularly if you are a beginning teacher.  Put yourself in the shoes of the employer.  Would you hire someone who intends to quit in three months?  Work permits are not available based on contracts of less than a year so you would technically be working illegally. 

Besides the contractual and legal aspects, you have to ask yourself if you are really being fair to the students.  There are idiosyncrasies in Vietnamese, both cultural and linguistic that effect your teaching methods.  You certainly don't have to learn Vietnamese but you need to be aware of how your students native language intersects with their learning of English.  You can't learn these things in any TEFL class and you probably can't even learn them in only three months of classroom work.

Hopefully you are not seeking to have your wages subsidize your stay.  Any new teacher should be cautioned to bring a few months of living expenses in case things don't work as planned.  Why not take the same money and have an extended vacation?