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 !