Python function that uses a word as the encryption key, rather than an integer

[wak_stephanie]

I'm supposed to create a program that uses a word as the encryption key that behaves like this in the end

$ python vigenere.py
Type a message:
The crow flies at midnight!
Encryption key:
boom
Uvs osck rmwse bh auebwsih!

My main issue is with the encrypting aspect of it.

def vigenere_cipher(plaintext, key):
encrypted_string = ''
for i in range(len(key)):
# loop over plaintext by index
    plaintextVal = ord(plaintext)
    # if character in plaintext is alpha do the following:
    keyVal = ord(key[i])
    keyVal = keyVal - 97
    # get alphabet position for character in key

    plaintextChar += chr(keyVal + plaintextVal)
    if plaintextVal >= ord('z')
    #get alphabet position for character in plaintext
        plaintextVal = plaintextVal - 26
        # rotate character from plaintext with a character from the key 
        # convert new position back to a character if you haven't done it already
        # concatenate new character to an encrypted string             


        print PlaintextVal

return encrypted_string

I'm getting a bunch of invalid syntax error throughout and I'm confused on how to go about fixing the code.

Thanks in advanced!