Vigenere Cypher - Code is not working as I expect - arrays

Question

I'm trying to figure out why my code is not iterating correctly on z. I would like to iterate every 1 z, but it seems like it's a bit random. Does anyone have any suggestions on how I might do this? If there anything else I'm doing wrong, please let me know. I think I'm really close. I'm also curious how I block spaces/other characters from being encrypted.
#include <cs50.h>
#include <stdio.h>
#include <string.h>
#include <ctype.h>
int shift(char c);
int main(int argc, string argv[])
{
// Check to see if two arguments are enterted at launch
int cipher = 0;
if (argc != 2)
{
// If not return error & return 0
printf("Usage: ./vigenere keyword \n");
return 1;
}
else
{
int strlength = strlen(argv[1]);
// Iterates through characters in second argument (key), checking to see
if they are digits
for (int k = 0; k < strlength; k++)
{
if (isdigit(argv[1][k]))
{
// If not return error & return 1
printf("Usage: ./vigenere keyword\n");
return 2;
}
}
//char *c =argv[1];
string plaintext = get_string("Plaintext: ");
int len = (int) strlen(plaintext);
//int b = atoi(c);
//char code[len];
//strcpy (code, plaintext);
int z=0;
for (int j = 0; j < len; j++)
{
int key = shift(argv[1][z]);
//printf("%i",z);
if (isupper(argv[1][z]))
{
//printf("theory\n");
cipher = ((((plaintext[j] - 'A') + key) + 'A'));
//cipher = ((((plaintext[j] - 'A') + key) % 26) + 'A');
//printf("%c", (((plaintext[j] - 'A') + key) % 26) + 'A');
//printf("%i",z);
printf("%c",cipher);
}
if (islower(argv[1][z]))
{
//printf("theory\n");
cipher = (((plaintext[j] - 'a') + key) + 'a');
//printf("%i",z);
printf("%c",cipher);
}
//z++;
//else if (!isalpha(plaintext[j]))
//{
//printf("%c", plaintext[j]);
//}
if (z > strlen(argv[1])-1)
{
z=0;
}
/* else
{
z++;
}
else
{
z++;
}
*/
//z++;
}
printf("\n");
}
}
int shift(char c)
{
int i = c;
if (i <= 'Z' && i >= 'A')
{
return ((i - 'A') % 26);
}
else
{
return ((i - 'a') % 26);
}
}