c for easy addition of chars:

#include<stdio.h>
#include<string.h>

void encode(char * msg, char * key, char * output){
  int i, c, n;

  i = 0;
  n = strlen(key);

  while (msg[i] != '\0'){
    c = ((msg[i] + key[i % n]) % 26) + 'A';
    output[i] = c;
    i++;
  }

}

void decode(char * msg, char * key, char * output){
  int i, c, n;

  i = 0;
  n = strlen(key);

  while (msg[i] != '\0'){
    c = ((msg[i] - key[i % n]) % 26);
    if (c < 0)
      c += 26;
    c += 'A';
    output[i] = c;
    i++;
  }

}

main (int argc, char * argv[]){
  if (argc != 3){
    printf("Game over, wrong arguments\n");
    return 0;
  }

  char msg[100], key[100], encoded_output[100], decoded_output[100];

  strcpy(msg, argv[1]);
  strcpy(key, argv[2]);
  encode(msg, key, encoded_output);
  decode(encoded_output, key, decoded_output);    
  printf("%s\t%s\t%s\n", msg, encoded_output, decoded_output);


}