Vayama has been great for CC fares - I noticed they've been consistently cheaper than what TK sells them for on their own website.

However, on neither TK not vayama can you mix CC and Y on the same ticket. I think I even had trouble trying to book JFK-IST-X, where no IST-X flights have a CC cabin. IIRC, neither Turkish nor vayama searches showed options with CC in JFK-IST.

Of course this is quite stupid. TK has made a clever but risky move in providing a product that isn't as expensive as business, yet is (allegedly) miles better than economy. If they want CC to be a success, they should at least make it easy for people to spend their money on it!