The Porta Cipher is very similar to the standard Vigenère Cipher, except that the keyword letters are paired up. That is, instead of having 26 possible alphabets, there are only 13, each used twice.
******* A B C D E F G H I J K L M N O P Q R S T U V W X Y Z
A or B: n o p q r s t u v w x y z a b c d e f g h i j k l m
C or D: o p q r s t u v w x y z a b c d e f g h i j k l m n
E or F: p q r s t u v w x y z a b c d e f g h i j k l m n o
G or H: q r s t u v w x y z a b c d e f g h i j k l m n o p
I or J: r s t u v w x y z a b c d e f g h i j k l m n o p q
K or L: s t u v w x y z a b c d e f g h i j k l m n o p q r
M or N: t u v w x y z a b c d e f g h i j k l m n o p q r s
O or P: u v w x y z a b c d e f g h i j k l m n o p q r s t
Q or R: v w x y z a b c d e f g h i j k l m n o p q r s t u
S or T: w x y z a b c d e f g h i j k l m n o p q r s t u v
U or V: x y z a b c d e f g h i j k l m n o p q r s t u v w
W or X: y z a b c d e f g h i j k l m n o p q r s t u v w x
Y or Z: z a b c d e f g h i j k l m n o p q r s t u v w x y
So if the plaintext said "This message is simply an example of the Porta cipher. The actual puzzle is something else." and if the keyword were "geocache", then the ciphertext would become "Dwpe zscdqvy wf eyomne oa shppass bt dwu ahdgo sxmwyd. Gvu pseaoy bekjny wf eloueowau unct."
More specifically, we have:
Geoc achegeo ca chegeo ca chegeoc ac heg eocac hegeoc...
+This message is simply an example of the Porta cipher...
Dwpe zscdqvy wf eyomne oa shppass bt dwu ahdgo sxmwyd...
But, the actual puzzle, is something else. Not even a cryptocache. You just need to find it. Somewhere.
P.S. Big thanks to klesieur for beta testing (and supplying some of the finer touches to this cache. (Translation: she knows that she is DQ'd for the FTF here.)