It's a secret (10)

So how do we use the Viginère cipher?  Well, we need a key word.  Ideally it will be a word with no "a"s and no repeated letters - so let's use "turkey".  Right, we take our message ("Put the kettle on back in ten minutes") and write our keyword over it again and again as often as necessary -

TUR KEY  TURKEY TU RKEY  TU RKE YTURKEY 

PUT THE  KETTLE ON BACK  IN TEN  MINUTES

and now we encipher the first letter (P) using the T line in the square - so we look P up on the top line and then follow it down to the T line, where it comes down as I. Then we encipher the next letter (U) using the U line and this comes out as O and the third letter T using the R line - and so on.  And our message becomes -

IOK DLC DYKDPC HH SKGI BH KOR KBHLDIQ

Now compare this with what we came up with in post 4.  There are five Es in the message, and back in post 4 they all came out as J - here they come out as C,Y,C O and I.  The double T in kettle came out back then as YY. here it come out as KD.  So frequency analysis won't be of any use here. Indeed this method of encipherment became known as le chiffre indechiffrable - the indechipherable cipher - and remained so for a couple of centuries.  But all good things come to an end...