Unless you have a good reason to activate types (forms in this case) based on a string, I would strongly recommend not to do this.
Using a string variable makes your code vulnerable to different kinds of errors, for example spelling mistakes in the string. These are not detected by the compiler since the form name is 'hidden' inside a string.
Personally I would prefer rethinking the need for this and possibly use generics instead. For example if you need to have a common method for all forms, you cound define a generic method that accepts any Form but only Form.
For information about generics, see Generic Types in Visual Basic