Covert Conversation - A North Daniels' Puzzle

Question

It's very important to keep your login details secure, which might entail not writing it down, encrypting them, or using a password manager. What you should not do is save them on some random USB and chuck it in your bag, because if you are, for example, a gang ringleader, someone might accidently dip their hand into your bag and fish it out.

Obviously, this is exactly the sort of thing D.S.I North Daniels loves doing - being sneaky. After tracking Simon Ukle for a few hours, he'd finally snagged a USB from his bag whilst distracting him with a pain au chocolat. After some digging, North found Simon's logins file, which contained his Wheresapp password. Brilliant...

However, after logging in, North was not greeted with oodles of juicy evidence, or any contact information - just one conversation with someone called Jude:

Simon: Are you there Jude?
Jude: Xon j af. Vhomo amo vo foopjgt?
Simon: Uagg gaol tplo lyx ikrcnl yka dc ba!
Jude: Qawgf gas jycfg?
Simon: Vs ldwyst aisoo dwop vssc - awlpqnrjf ykqn lrsaso vwo swoy lswoy
Jude: Qcwfb fca bapirl?
Simon: Kspqsmnic np ijq wffjvst ji nq'p jvi - jify ni psqp. Hfwab wit vonqs qnsp
Jude: O mytrhfjpyt
Simon: We geep ip i opipan page - yjq otiff hjp ue fipe. Wiaopnjipo melqamed
Jude: Dbiad
Simon: Lm, ft's Sfkml!

It seemed like the conversation was corrupted in some way, or maybe it was encoded. Simon took after his sister Jane - he always took things one step further

North whipped open his laptop, and after some speed-coding, the conversation was revealed...

Yes...no?

Well, it looked like gibberish, but North never had the knack for cryptic crosswords. It seems as though Grandma Daniels' skills may be useful after all...

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What's the FULLY decoded conversation?

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Hints

1. Simon says look at the four responses

2. Simon says watch out for any purpose

3. Simon says lets play charades

4. Do not look in other North Daniel puzzles for locations

Answer 1

_{A partial answer - message decoded, but can't quite suss what to make of the output in order to reach the final answer...}

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The way to decode this message is to use:

a different substitution cipher for each line. Specifically, each line's cipher key is constructed by reducing the previous line's letters to only the first appearance of each letter in its plaintext. Match these up to A, B, C, etc. and when you run out of letters from the previous message, just list the remaining unused letters of the alphabet in their usual alphabetical order. For example...

The first line of the message is "ARE YOU THERE JUDE?" Reducing this to only the first appearance of each letter yields "AREYOUTH...J.D." Bunch these together and add on all the unused letters of the alphabet and your key for the next line of text is AREYOUTHJDBCFGIKLMNPQSVWXZ.

The message, therefore, translates as follows:

Simon: ARE YOU THERE JUDE?

Jude: Xon j af. Vhomo amo vo foopjgt?
KEY: AREYOUTHJDBCFGIKLMNPQSVWXZ
Plaintext = Jude: YES I AM. WHERE ARE WE MEETING?

Simon: Uagg gaol tplo lyx ikrcnl yka dc ba!
KEY: YESIAMWHRTNGBCDFJKLOPQUVXZ
Plaintext = Simon: WELL LETS JUST SAY DRINKS ARE ON ME!

Jude: Qawgf gas jycfg?
KEY: WELTSJUAYDRINKOMBCFGHPQVXZ
Plaintext = Jude: WHATS THE FIRST?

Simon: Vs ldwyst aisoo dwop vssc - awlpqnrjf ykqn lrsaso vwo swoy lswoy
KEY: WHATSEFIRBCDGJKLMNOPQUVXYZ
Plaintext = Simon: WE PLAYED CHESS LAST WEEK – CAPTURING YOUR PIECES WAS EASY PEASY

Jude: Qcwfb fca bapirl?
KEY: WEPLAYDCHSTKURINGOBFJMQVXZ
Plaintext = Jude: WHATS THE SECOND?

Simon: Kspqsmnic np ijq wffjvst ji nq'p jvi - jify ni psqp. Hfwab wit vonqs qnsp
KEY: WHATSECONDBFGIJKLMPQRUVXYZ
Plaintext = Simon: PESTERING IS NOT ALLOWED ON IT’S OWN – ONLY IN SETS. BLACK AND WHITE TIES.

Jude: O mytrhfjpyt
KEY: PESTRGINOALWDYBCKHFJMQUVXZ
Plaintext = Jude: I UNDERSTAND

Simon: We geep ip i opipan page - yjq otiff hjp ue fipe. Wiaopnjipo melqamed
KEY: IUNDERSTABCFGHJKLMOPQVWXYZ
Plaintext = Simon: WE MEET AT A STATIC TIME – YOU SHALL NOT BE LATE. WAISTCOATS REQUIRED

Jude: Dbiad
KEY: WEMTASICYOUHLNBRQDFGJKPVXZ
Plaintext = Jude: ROGER

Simon: Lm, ft's Sfkml!
KEY: ROGEABCDFHIJKLMNPQSTUVWXYZ
Plaintext = Simon: NO, IT’S SIMON!

So, now that the message is decoded, what do we do next? I'm not entirely sure, but worth noting that:

we have not yet used the geography tag - presumably what we have here are some clues to help us work out the location. As to what the 'first' and 'second' are, I am not yet entirely sure...

Next step needs some thought - or another eye to spot something I cannot!