PYTHON 3 Vigniere decoder issue; even with code outputting spaces and punctuation correctly, after punctuation mark the decoder doesnt work

Question

Working on a Vigniere Decoder in Python 3 but after processing punctuation marks (which output correctly) the decoding algorithm is thrown off, I assume this to be due to incorrect drawing of comparative index values (between encoded message string and repeated key phrase string).

To get around this I thought of including a punctuation counter for correction, but I do not know where in the maths of my nested if loop to implement this. Or if this (punctuation counter) is even a fix that will work in the method of logic I am using.

I'm only working with basic tools in Python as this is supposed to be a challenge of what I've learnt so far, so simpler/more advanced ways to program a Vigniere decoder might not be something I know, in case anyone sees my attempt and thinks what the hell are they doing!

import string
alphabet = (string.ascii_lowercase)
punctuation = (string.punctuation)

def vignieredecoder(message, key):
    keycompare = ''
    index = 0
    decodedmessage = ''

    #makes key repeat to length of message w/ spaces integrity the same
    
    for character in message:
        if character == ' ':
            keycompare += ' '
        else:
            keycompare += key[index % len(key)]
            index += 1
    key = keycompare
    

    # decoding loop

    for characters in range(len(message)):
        punctcount = 0
            
        if message[characters] in punctuation:
            decodedmessage += message[characters]
            punctcount += 1
                    
        elif message[characters] == ' ':
            decodedmessage += ' '
            
        else:
            # store index value of character in message, e.g. a = 0, b = 1, etc
            messageletterindexval = alphabet.index(message[characters])
            
            # store index value of character in keycompare[place of character in message], e.g. if key is friends and first iteration being 0 = f
            keycompareindex = key[characters]

            # access character in alphabet at indexvalue of keycompare, e.g. if key is friends and first iteration of for loop, finds f in alphabet and returns indexval 5
            keycompareindexval = alphabet.index(keycompareindex)
            
            
            decodedletterval = messageletterindexval - keycompareindexval
            
            if decodedletterval < 0:
                decodedletterval += 26
                decodedmessage += alphabet[decodedletterval]
            else:
                decodedmessage += alphabet[decodedletterval]
    return decodedmessage

print(vignieredecoder('dfc aruw fsti gr vjtwhr wznj? vmph otis! cbx swv jipreneo uhllj kpi rahjib eg fjdkwkedhmp!', 'friends' ))

terminal output: you were able to decode this? rzmp jcao! zjs bor wfxmnfab rpgub gcf zvqbeo bo asvgjhmyqel!

expected output: you were able to decode this? nice work! you are becoming quite the expert at crytography!