Is there some kind of non disclosure agreement between Fiverr and vendors to stop (eg. website designer) going behind my back directly to my client since they need current website details, logins etc?
Nope, nada, zilch.
Use at your discretion.
However, there’s a strict policy of sharing PERSONAL INFORMATION, such as email, skype id etc. on Fiverr unless absolutely required for rendering service to a Buyer.
Fiverr is the platform, not the service provider, and sellers don’t work for Fiverr, they merely use the platform, so no, Fiverr doesn’t put in place any controls such as NDAs.
If you want to be sure, you can get the seller to sign an NDA. You’re only allowed to exchange personal information, and it has to be essential to completion of the gig, through an order. So if you do get a seller to sign an NDA, you must first order from them, then the first business must be to sign the NDA.