Would Vietnam accept me as a junior mechanical engineer?

If your Vietnamese includes both technical as well as basic fluency, you probably can find work.   If not, you would have to get up to speed on technical vocabulary.

Just for your information, one of my first neighbors in Vietnam was a young man who was an electrical engineer, well employed in a firm with high rise work in Ho Chi Minh.  He was making 18 million a month working 6 days a week while I was making 20 million working as an English teacher part time.  The engineer wages may be higher now and ESL wages seem flat but perhaps you could likely do better.  Per you bio you may be Viet Kieu which could mean some discounting of any ESL wages.  Yes, it is unfair but real.

Either route will probably not do your resume a lot of good on return to Canada.