Im sure this was mentioned somwhere before, but most of the npcs text is in their .msg file.
Vics is dcvic.msg
Cassidy's is vccasidy.msg
Sulik's is klsulik.msg
You can edit these with notepad stc.
But if you want to entirly change what the say when and this affects the game, you'll need to edit their script file which a script compiler/decompiler.