If we are talking about a double cipher (meaning the plaintext is ciphered, then ciphered again with a different algorithm) we need:
Cipher Algorithm #1
Passphrase and or Keys #1
This results in "plaintext #1" which becomes the "ciphertext #2". Then you need:
Cipher Algorithm #2
Passphrase and or keys #2
Which should result in "plaintext #2", which is the original text.
The ciphertext #1 is "yyuahrnqyf hcgi vxmly navgu ulli xmixo. 551961." includes the hints "16" (The thread title), a quote from Julius Caesar, and the blatantly capitalized "MERCHANDISE"
So assuming the Cipher Algoritm #1 is a Keyed Caesar, the shift is 16 (or 10?) and the key is MERCHANDISE, the resulting "plaintext #1" is either:
iiepomqciv onws fhkzi qpfwe ezzs hksha. 551961.
ookvuswiob utcy lnqfo wvlck kffy nqyng. 551961.
The speculation right now is that Algorithm #2 is a Vignere, because of the Giovan connection. Alternatively it could be one of Giovan's algorithms. EDIT: However, Giovan's algorithms (at least the first one) doesn't include the letters k or v, both of which are in the ciphertext #2. So that rules out Giovan's first algorithm. That is, unless you modernize the algorithm to include k and v, but I think that's a stretch...
Any other ideas?