In the absence of VM publicly releasing figures (and let’s be honest, they won’t), the usual assumption is that VM and all other broadband suppliers actually make a loss or at least just about break even on the initial ‘new customer’ prices, but they know that the bulk of their customers won’t leave at the end of the promotion period and that is where they make their money.
Now, be honest, as a new customer you have a choice, £x for 18 months as an introductory price, or another supplier saying it will cost you, say, twice that but we won’t increase it after the 18 months.
The vast majority will opt for the first, and, frankly, anyone who says they won’t is either a) lying or b) a complete idiot!
Bottom line, if VM (or any other ISP) were to say ‘this is the price for the service, no ‘introductory discount’ but it won’t enormously go up after 18 months’, they’ll be out of business within a couple of years.