I personally would teach it no differently than how one teaches mathematics: provide a set of rules, and how to make use of them step-by-step starting with the most basic of rules, and building in complexity as you move forward. Once a foundation has been built, you can than move into the exceptions to grammatical rules.
In a vocational school I think it would be important to stress why proper grammar is important in any vocation. If you can apply the usage of proper grammar to success in the workforce your students will likely be more motivated to learn it. You can start with the importance of proper grammar in a cover letter and resume in securing a good job.
The cause of their poor grammar probably stems from learning English in a school that stresses the whole-text approach to learning to read. Students who learn to read in this way are usually poorer in spelling and grammar than those who learn to read using the basic skills method. Like in mathematics the best way to teach proper grammar after the fact is to break language down into it's component parts dissecting it out of context, and then putting it back together in context.