C function to move and collapse cells in 2048 game
Don't repeat yourself

The two loops can be joined pretty easily. We basically just need to factor out the (few) pieces that are different between the two, and pull those pieces out of the loop. The loops are just short enough, however, that the result may not be a whole lot shorter than the two loops are right now (so if you care more about short code than lack of repetition, you may not want to do this).

Comments

Right now, you've provided comments for what the loop does in one of the loops, but not the other. By collapsing the loops together, we can eliminate this inconsistency. That leaves the question of whether to include the comments or not. In general, I'd rather write the code to be self-documenting instead. See below for at least some of how to do that.

Symbolic names

Instead of writing the code to explicitly state the an empty cell contains the value 0, I'd prefer to give that a name:

#define EMPTY 0

// ...

if (_vector[i] == EMPTY)
   // ...


This (probably) eliminates any requirement for a comment to tell us that we're checking whether that cell is empty.

Unnecessary Indirection

It appears that you can pass _score as a pointer to int rather than a pointer to pointer to int as you're doing now. At least as far as I can see, the extra level of indirection isn't accomplishing anything useful at all.

Bool Parameters

I generally discourage using bools as function parameters. I'd rather use an enumeration that directly states what you want to express:

enum { UP, DOWN };

collapseVector(..., UP);


...seems much more meaningful than:

collapsVector(..., true);


Final Code

One possibility would be something on this general order:

#define EMPTY 0

bool down(int a, int b) {
    return a &lt; b;
}

bool up(int a, int b) {
    return b &lt; a;
}

enum direction { UP, DOWN };

/**
* collapseVector: Collapse a vector "2048" style, flush the cells in one direction, joining them if legal.
* @param _vector 1-D array to be collapsed
* @param _score Score counter
* @param invert Whether or not to inverse-flush vector. Useful for Up(false) vs. Down(true), and so on
*/
void collapseVector(int _vector[MAX_BOARD_POS], int *_score, direction dir) {
    int idx = 0;
    int buf[MAX_BOARD_POS] = { 0 };
    int start = 0;
    int end = MAX_BOARD_POS;
    int inc = 1;
    bool (* cmp)(int, int) = down;

    if (dir == UP) {
        idx = start = MAX_BOARD_POS - 1;
        end = 0; // Not sure if this is quite right, but I think it's at least close
        inc = -1;
        cmp = up;
    }

    for (int i = start; cmp(i, end); i += inc) {
            if (_vector[i] == EMPTY) {
                continue;
            }
            if (buf[idx] == EMPTY) {
                buf[idx] = _vector[i];
            }
            else if (_vector[i] == buf[idx]) {
                ++buf[idx];
                *_score += 1 &lt;&lt; buf[idx];
            }
            else {
                buf[idx += inc] = _vector[i];
            }
        }
    }
    memcpy(_vector, buf, MAX_BOARD_POS * sizeof(*_vector));
}