Warning: X may be used uninitialized in this function

one has not been assigned so points to an unpredictable location. You should either place it on the stack:

Vector one;
one.a = 12;
one.b = 13;
one.c = -11

or dynamically allocate memory for it:

Vector* one = malloc(sizeof(*one))
one->a = 12;
one->b = 13;
one->c = -11
free(one);

Note the use of free in this case. In general, you'll need exactly one call to free for each call made to malloc.

This might not be the most professional solution, but instead of initialising it using malloc, you can also initialise it using new:

Vector *one = new Vector();

I personally find it more elegant.

You get the warning because you did not assign a value to one, which is a pointer. This is undefined behavior.

You should declare it like this:

Vector* one = malloc(sizeof(Vector));

or like this:

Vector one;

in which case you need to replace -> operator with . like this:

one.a = 12;
one.b = 13;
one.c = -11;

Finally, in C99 and later you can use designated initializers:

Vector one = {
   .a = 12
,  .b = 13
,  .c = -11
};

When you use Vector *one you are merely creating a pointer to the structure but there is no memory allocated to it.

Simply use one = (Vector *)malloc(sizeof(Vector)); to declare memory and instantiate it.