Thought I'd share my experience last week with Delta and Velocity. My better half and I (both Virgin Gold) took advantage of some on sale SYD-LAX premium economy seats with Virgin. Once the impulse to buy the tickets was over we decided we really didn't want to goto LA, it just doesn't appeal, so we booked LAX-SFO on Delta using our velocity FF number on the booking.
After reading this thread we were ready for a lot of explaining to try to get our Virgin Gold privileges recognised, so we went armed with lots of screen shots on my iPad from both Virgin and Delta sites to help us through if we came across Delta staff who weren't educated in the finer details. I must say they really weren't needed, everything went smoothly, we received all privileges our Virgin Gold status afforded us - free baggage, priority check in, priority baggage, free upgrade to Comfort+ <24 hrs (done via the Delta app & website when checking in) and lounge access in both LAX and SFO. My FF & status points for the Delta flights were credited to my Virgin account in less than 48 hours - I actually got the LAX-SFO points before Virgin credited me with their SYD-BNE-LAX points. The only hiccup we had, if you could call it that was at the SFO lounge, the attendant asked me for my AmEx Platinum card for entry, when I asked why the other attendant stepped in, looked at our boarding passes and said "no that's fine, they are Virgin Gold" - I can't remember if we had to present our cards as proof.
This was our first time in the States and first time flying with Delta, I have to say it was a wonderful experience, the staff were all friendly and courteous, the aircraft were almost new (think we were just lucky there), the in flight meal/snack was far and above anything Virgin or Qantas have ever presented and was served promptly, it had to be, it was only a 53 minute flight! The lounge and the lounge staff were A1, the entry attendants both in LAX and SFO explained the lounge to us, where everything was and how to sign up for showers if we needed - we really must have looked like grotty newbies