26125

As with the previous challenge, I ran this through my Vigenere solver - and I was surprised to see the best key was "PI" (an anagram of the challenge title). Then I could undo the transposition.
CONGRATULATIONSIFYOUCANREADTHISTHISENCRYPTIONSGETTINGQUITECOMPLICATEDTHEVIGENEREMETHODOFENCRYPTIONCOMMUTESWITHATRANSPOSITIONIFITSROWANDCOLUMNLENGTHSAREONEMORETHANAMULTIPLEOFTHEKEYWORDLENGTHSOINTHISCASEWITHAKEYWORDOFLENGTHWOWENEEDTHEMTOBOTHBEODDIHOPEYOUVEENJOYEDDECRYPTINGTHESEMESSAGESWHYNOTTRYCOMINGUPWITHSOMECIPHERSYOURSELFANDTESTINGYOURFRIENDS

I brute-forced the transposition, going over the first 30 Caesar squares.