Can we get rid of the fixed copy buffer size and make it dynamic? It seems like a waste of memory to have this always, and an unnecessary limitation.

The variable names can also be more descriptive, copybuf doesn't say much.

Can you move these inside the if(), and free buf in there too instead of setting it to NULL?

We've also got C99 now so no need to define things at the start of the function.

Check for malloc failure.

Also not sure why you use len_utf8 + sizeof(wchar_t) here, just len_utf8 + 1 should be enough?

I don't think an error needs to be reported for this, all apps I know just silently ignore this.

Same comment as above.

Would rather not store clipboard here (blender has many clipboards, and seems a bit arbitrary to store this particular clipboard in 'Global').

Conventions from BKE_mask_clipboard_*, BKE_tracking_clipboard_* can work here.

static structs can be access by function if its needed.

BKE_vfont_clipboard_get(wchar_t *r_buf, int *r_buf_len)

Variables could be made more clear, (strp, buf, len)...

While not really confusing, would rather make it more clear whats going on with taking buffers from different sources and comparing.

eg:

/* Store both clipboards as utf8 for comparison,
 * Give priority to the internal 'vfont' clipboard with its 'CharInfo' text styles
 * as long as its synchronized with the systems clipboard. */
struct {
    char *buf;
    int len;
} clipboard_system = {NULL}, clipboard_vfont =  {NULL};

... so logic for checks reads more straightforward.