Have you been given a tracking number?
I had the exact same problem with Yodel, they simply didn't turn up on the scheduled day.
So I tracked it online and found out it had been delivered to a neighbour. Checked both neighbours, no sign of the modem. Then called up yodel and they told me it had been delivered to a "neighbour" about 40 doors down. No sign of a card either so I would never have found the parcel had I not called them up.
So yeah check tracking info and try give yodel a call in the morning.
Also in regards to your question on why does Virgin Media use Yodel. Companies generally use cheapest couriers in order to maximise take home pay for executives. So cheaper the courier used, the more money the executives get to take home. So if you need to send 500 modems a year using the cheap option it may cost £2 a pop, that's £1000 on modem logistics for the year. But if you use a proper courier like DPD or UPS it may cost £6 per modem, that equates to £3000 per year. That means a lucky executive can pocket a £2000 a year bonus just for using a cheaper courier. Obviously the bonus amount will vary depending on the actual shipping volumes but I hope that answers your question.