Only if you work for West Coast!
I have always (okay both times) been charged weekend first when travelling on VWC with a Priv because I work for a 'lower' TOC, and yes one of them did say that!
I've probably roughly travelled with VT, as many times as XC whilst using my Std Priv to buy a priv off peak ticket.
In my experience... XC have charged me roughly 75% of the time. Excluding one "incident", whereby, upon leaving Wolverhampton on a Saturday afternoon in an extremely quiet First Class carriage (on a 220/221) and presenting my priv return, whilst asking to upgrade, the TM in no uncertain terms told me that I should have told him that I wasn't a First Class ticket holder when I saw him by the carriage door. He was one of the rudest members of staff I've ever encountered.
I once was travelling between Bristol Temple M and Birmingham New Street and eventually got charged to upgrade, as we passed through Longbridge- the TM had been having issues with her machine.
VT on the other hand have only ever once charged me (Wolverhampton based TM I believe)- On an admittedly busy Class 221 working, however, I didn't appreciate his comment of "everybody's got to pay", after I (tongue in cheek) mentioned that it was the very first time, out of dozens of journeys on VT, that I've ever been charged. They then went on to later on in the journey refrain from charging staff from another TOC, as they knew them. I've even had a few Wolves VT TMs jokingly say to me "don't be silly" as I offered to pay them to upgrade before this journey, so I suppose it just depends on how kind the TM is.