Now that is interesting, what is the most common cause?
Again, in my own experience ....... set-top boxes. Where I live there are strong Asian/Polish communities, and they have their own 'Chipped' boxes that seem to be a major problem. Don't get me wrong, there's the 'Chipped' SKY boxes as well that emanate REIN due to cheap electronics utilised.
I'd say by far and away, these account for 70-80% of the cases I've attended. The other's have included cheap LED lighting, TV's and a rather tricky one that turned out to be a small fish-tank's flu-tube light, that was only turned on from time-to-time. If you pardon the pun ........ one gets a buzz (Audible REIN noise on 612kHz), when the source is finally found !!
It ain't the chipped boxes per say ( I've fixed enough with both internal and external PSU's so cheep i'm amazed they actually work properly with the floating earth and ripple), I've personally narrowed it down even further, It's the cheap chinese power supplies and it stretches to a lot modern electronics.
That said, when I investigated my own rein issue I tracked it down to my next door neighbor, and his TV, coupled to a sky HD box via HDMI cable, then transmitting it up the phone line from the boxes built in modem. I offered to fix the offending device and re-cap the PSU before it inevitably blows , he refused. I said don't ask me to fix it when it blows, it blew the rein was gone, he asked me to fix the TV and I said NO, you missed the boat mate.
Also your spot on about rein affecting ADSL more than VDSL, I suspect it less about frequency( I've seen powerline ethernet cause rein at higher VDSL frequencies).
I had the rein before and after I switched to VDSL however there are a few reasons why VDSL is less prone.
the BT Openreach VDSL faceplate filters which I fitted to try to help the ADSL rein made quite a difference even to ADSL2+, I gained 3mbps in my case with the openreach VDSL faceplate (mk1 at the time) over known good microfilters. With openreach VDSL faceplates and managed installs being standard until recently with VDSL, where as most ADSL self installs with connections use Microfilters, i Think this has been a big factor too.
Also because of the extra distance involved with ADSL, by the time the signal gets to your home the signal is far weaker, attenuated and prone to being affected by noise, than the VDSL signal which is generated alot closer to home than the exchange in most cases. This was clear and obvious when I switched to FTTC in the DSL stats, even with the rein at the I had an attainable rate of 94mbps so it wasn't affecting speeds at the time, even in the rein region the SNR on VDSL is vastly better, it would just load up the bit's elsewhere anyway.