It's not a change from CPI to RPI, because in previous years VM simply increased by whatever they thought they could get away with, but there was nothing baked into the terms and conditions. Under Ofcom's rules, if it wasn't in the T&Cs and the increase exceeded CPI, then VM or any other ISP had to give customers the option to leave.
The big change here is that VM are baking this in contractually so it can't be a grounds to leave, which means in future customers will normally be hit by two of these swingeing annual increases in an 18 month fixed term contract.
So that's why you get the chance to leave this year, both due to the above CPI increase, and because it's a material change to terms. But next year, because it will be baked into the T&Cs you won't have the option to say no. There's no magic around CPI or RPI here, the T&Cs could say "prices each year will rise by the number of Boris Johnson's illegitimate children plus 5.1%" and it would still qualify as enforceable and not giving you the chance to refuse (because Ofcom are as clueless as Ofgem).