Vigenere - is there a simpler logic behind it?

Question

My code for vigenere.c works for check50, but ends up being much longer than caesar.c in the encipher part.I was wondering if I was missing a simple point and have meandered through the logic behind it? I couldn't find another better way to avoid jumping to the next element of key shift if the character is not alphabetic!

I'm sure there is a less messy solution to vigenere?

Here is the encipher part of my code for vigenere.c

int i = 0;
int l = -1;

do 
{
if (isalpha (p[i]))
    {
        do{
              l++;
              if (isupper (p[i]))
              {
              printf ("%c", (p[i] - 65 + k[l%m])% 26 + 65);
              }
              else
              {
              printf ("%c", (p[i] - 97 + k[l%m])% 26 + 97);
              }
              i++;
        }while (isalpha(p[i]));
    }

    else
    {
        printf ("%c", p[i]);
        i++;
    }
} while ( i < n);