Breeze was too conspicuous
Kelsier specifically needed someone to pose as Renoux's heir. His original plan was Dockson, but he was too dangerous, having drawn attention to himself previously.
“Can’t be me,” Dockson said. “I got marked during the Eiser job just a
couple months back.”
“What?” Yeden asked. “Do I want to know what they’re talking about
“He means that the Ministry is watching for him,” Breeze said. “He
pretended to be a nobleman, and they found out.”
—The Final Empire
He couldn't have someone assume the role of a random noble: it had to be someone with a plausible reason to be associated with Renoux. Basically, an heir: a niece or nephew.
Breeze was indeed an excellent candidate. However, he was so good that he had already done exactly the same thing several times before.
“I’m out as well,” Breeze said. “I already have several aliases among
the nobility. I suppose I could use one of them, but I couldn’t go to
any major balls or parties—it would be rather embarrassing if I met
someone who knew me by a different alias.”
—The Final Empire
Had Breeze attempted to pass himself off as Renoux's nephew, his ability to spy would have been severely limited. Kelsier considers the other crew members, but discards them as possibilities for various reasons.
Using Vin, on the other hand, had a number of benefits.
She was around the right age.
While she lacked Breeze's social skills, she could attend a wide range of events without risking capture.
Perhaps most important, she was a Mistborn. She thus possessed Breeze's Allomantic talent, plus more than a few others. This would be a tremendous advantage if things ever came to a fight. Indeed, the first thing Kelsier did after their meeting was over was to instruct Vin in the use of the various metals.