Bonjour Peter !
The reason is one of pronunciation :)
In the case of "déçu", we want to preserve the pronunciation of the root of the verb "décevoir", i.e. [ss], hence the use of the cedilla in front of "u".
As for "vécu", there's no such imperative, so we just use "cu", which is pronounced [ku].
I hope that's helpful!À bientôt !
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