Ive just read the website again and can't see the reference of multiple entries using visa exempt so it may have changed when they changed it from 15 to 45 days. Regarding Hoi An I was meaning going there instead of Chiang Mai if borders were a problem.
There is a limitation on how soon you can re-enter. Check the UK website for Vietnam entry requirements. It might be better to fly down to Hoi An which is so nice.