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Typo: specified.

Typo: specified. init_##vec_name##_c and append_##vec_name, I assume you also meant to make them static.

Instead of giving your users the ability to create any vec_name, I would consider, eg vec_##name, to give my vector types a naming consistency.

allows moving functions around to get one-point-of-failure, for example,

Instead of giving your users the ability to create any ##vec_name, I would consider, eg vec_##name, to give my vector types a naming consistency.

allows easily moving functions around to get one-point-of-failure, for example,

On my computer, the code works for one re-allocation, then it segmentation faults. It's easier to debug when the memory allocation is in one function instead of both init_##vec_name##_c and append_##vec_name. Suggest a feature of realloc,

On my computer, the code works for one re-allocation, then it segmentation faults. It's easier to debug when the memory allocation is in one function instead of both init_##vec_name##_c and append_##vec_name. A feature of realloc,

allows moving functions around to get one-point-of-failure, for example,

static enum error resize_##vec_name(struct vec_name* vec, size_t min_capacity) { \
    size_t new_capacity; \
    type* new_items; \
    if (!vec) \
        return NULL_POINTER_ERROR; \
    for (new_capacity = vec->capacity > DEFAULT_CAPACITY \
        ? vec->capacity : DEFAULT_CAPACITY; \
        new_capacity < min_capacity; new_capacity *= EXPAND_RATIO);\
    if (vec->capacity >= new_capacity) \
        return NO_ERROR; \
    new_items = realloc(vec->items, new_capacity * sizeof *new_items); \
    if (!new_items) \
        return MEMORY_ERROR; \
    vec->items = new_items; \
    vec->capacity = new_capacity; \
    return NO_ERROR; \
} \

Then this simplifies init_##vec_name##_c,

vec->length = 0; \
vec->capacity = 0; \
vec->items = 0; \
return resize_##vec_name(vec, capacity); \

and append_##vec_name,

if((e = resize_##vec_name(vec, vec->length + 1)) != NO_ERROR) \
    return e; \
vec->items[vec->length++] = item; \

Viewed as a state machine, your programme has some invalid states. Suggest in free##vec_name,

vec->capacity = vec->length = 0; \