The decision to make procedures not able to return value makes sense, however, it would be very convenient if you could write reusable procedures that don't need to be altered for multiple spots in the same program or different programs. I ran into this issue when I wanted to write a general purpose numerical integration procedure, where you could pass a variable to it, bounds, a precision value, and an expression, and it would set the given variable to the result of the integration.
So I suggest using a C-like approach and allowing procedures to accept a pointer to a variable so that you can pass whatever variable you want to it and it will alter its value.
I also suggest (I think this qualifies as the same suggestion) adding pointers to expressions (like function pointers) to facilitate the above type of programming.
I think that this would not add too much complexity to vizzy (and users who do not want to use it could just ignore the extra types), but would allow for greater code reuse and more effective sharing of programs.