After quite a bit of experimenting the only way I can use the Proxy files created by the FS5 in Vegas Pro is to 'rename' all the proxies so they match up with the mxf clips.
I have read on many other forums about people struggling to use the Proxies created by the FS5.
If Sony is listening this needs to be addressed immediately. The simplest solution would be to drop the 'S03' which gets automatically tacked on to the file name...: i.e. Clip0001S03.mp4 The S03 is unnecessary and causes issues with many editing systems out there which require the proxy clip name to match the full resolution file exactly....in the above case the FS5 full resolution file name would be: Clip0001.mxf so the Proxy clip needs to be Clip0001.mp4
And yes---I've sent a note to Sony but have not received a response.