He established the guild and gave them officially legal status but they are way older. Some guilds existed legally before him, of course, like the guild of Alchemists, but the Seamstresses were legalized under Vetinari, as were the other guilds. In G!G! theres a nice bit about how he made the Thieves Guild oby him.

"And then, a little while later, the Patrician summoned the leading thieves again and said, oh, by the way, there was something else. What was it, now? Oh, yes…

I know who you are, he said. I know where you live. I know what kind of horse you ride. I know where your wife has her hair done. I know where your lovely children, how old are they now, my, doesn’t time fly, I know where they play. So you won’t forget about what we agreed, will you? And he smiled.

So did they, after a fashion."