The best way not to forget what you learned in a Vajra Dance course is to repeat the steps many times. If you don’t have the chance to practice on a ‘real’ mandala nearby, you can paint or print a piece of a mandala to train single sequences of steps. You can also make a string mandala, which can be easily used outdoors (for indications on how to make one, please write to firstname.lastname@example.org). Or you can just visualize the mandala and practice the steps at home.
It is not necessary that these training mandalas be of the correct measures: you can adapt them to your space. Obviously dancing on these training mandalas doesn’t have the same function and benefit as dancing on a real mandala.