Type agnosticism, data structures and C

/prog/, I've been working on a C library with basic data structures and useful functions, and have implemented things like doubly-linked lists and vectors and so on. Now I'm working on Unicode-based string support. Here's my issue.

I've implemented type agnosticism on the vector type using void pointers (not preprocessor macros), so I want to use the vector type to represent a string. Each Unicode codepoint is an int32_t. Of course, I don't want to have to allocate four bytes on the heap for each codepoint, then point to that from each vector element; that's utterly stupid. On the other hand, doing typecasts to shove an integer in the void * type is kludgy and works very poorly, often requiring double explicit casts; once to expand/shrink the type's size, then once more to explicitly cast to a pointer or back.

How can I solve my issues and use my vector type for strings?

Don't write in C.