Vimala is less a handwriting program than a path for soul development. This is the essence of why it is not a good choice in the classroom. Vimala causes problems in both the academic curriculum and the child-development curriculum.
Academic Curriculum: Vimala is Not Good Handwriting
The purpose of handwriting is to facilitate written communication. When choosing a handwriting program, there are two areas to consider: 1) ease of writing; and 2) ease of reading. While the shapes of Vimala letters are not difficult to learn to form, some of them are very difficult to read. When children write Vimala, it makes their developing writing look even messier than usual. The lowercase letter G looks like it is not completely formed, even when written correctly. Vimala has letters of odd sizes, such as the lowercase letter T, which looks like an uppercase T. People who use Vimala look like they cannot capitalize properly. You can tell that that is not the case when confronted with perfectly formed Vimala written by an adult, but with students, Vimala just contributes to an aura of bad handwriting and error, particularly if the students already struggle to capitalize correctly.
Child-Development Curriculum: Vimala is Developmentally Inappropriate
It is very desirable for adults to take up their own development, such as by changing their handwriting. It is not desirable for teachers to take their students through such a path of self-development. Students are not developmentally ready to take up their development in this way until they come of age. Students should be working on the development of their etheric body and then astral bodies after puberty. Steiner is very clear on the developmental stages and ages. Only after this work is done are people ready to take up their development in freedom.
Teachers who bring Vimala are doing something very much against the spirit of Steiner's indications re: child development. Everything at the right time. Teachers must tread very carefully in this realm.
Form drawing is a better approach for the child-development curriculum. Steiner made form drawing for students. It helps the students work on the levels they are developmentally ready for. Adults who wish to take the practice of Form Drawing further are invited to consult Laura Embrey-Stein's work on Form Drawing for adults. She is quick to tell teachers never to bring adult form drawing into the classroom–students are not developmentally ready for it. Instead, Laura gives form drawing exercises that are developmentally appropriate for different ages/grades. She recognizes the power inherent in Form Drawing and uses it to work in tandem with child development.
In a nutshell, that is what is wrong with Vimala–it is for adults, not students.
My recommendation is to teach the students a standard cursive font such as D'Nealian, Handwriting without Tears, Zaner Bloser, etc. That way, everyone can read their handwriting because it is standard. If students wish to take up their development through handwriting when they come of age, they are then free to do so. They can use Vimala as a path of development when they are of an age to make those decisions for themselves.
If you want to help your students, a better path is to teach an academically robust Waldorf curriculum instead of Vimala. The Waldorf curriculum works on both the academic curriculum and the child-development curriculum. (Consult my book Continuing the Journey to Literacy for more information about how to make an academically robust Waldorf curriculum.) The Waldorf curriculum will help the students develop in a way that is healthy. Vimala won't–not until the students come of age and not unless they freely make the decision to take up their development. Such a path is fine for adults to take, but it is inappropriate for children.
For another view of why Vimala is not a good choice, go to The Parenting Passageway: https://theparentingpassageway.com/2013/01/27/cursive-writing/. While I do not agree with everything stated in this post, it is worth considering because there is a lot of wisdom in it, and it will help you flesh out this topic and consider it from different angles.
No one has full possession of the truth. If you look at the issue from multiple perspectives, you will be able to make a well-informed choice. Good luck and thank you for asking!