By its very nature VPN traffic is "hidden" from the ISP so the VPN traffic should not make any difference to the choice.
Of course both BT and SKY will have a "small" amount of VPN traffic used to hide P2P usage. SKY allow P2P without shaping, BT do not.
But at price, availability and make a choice. If you are already on SKY it seems logical to stay with them, same is probably true with BT.
Horses for courses.
Ex <n>ildram , been to SKY MAX - 15,225 Download
BE Unlimited - 21,000 Download 1,200 Upload ON THE LINE THAT SKY COULD ONLY PROVIDE 15,255 DOWN AND 800 UP ON!!!,
Moved house, now BE Unlimited 6,500 Down, 1Mb/s up - gutted!
FTTC Cab installation commenced 12th April - expect full 80 / 20 - bye bye BE, hello BT Infinity, install date Friday 10th August!