In the wake of the original 'vodafail' debacle, Vodafone took the cheapest option available to it to improve its basic signal reception and distance coverage in their otherwise most vodafaily-affected areas - namely, the rollout of its 850MHz service.
Vodafone has no 700MHz spectrum access (unlike Telstra and Optus) and hence has decided to 're-purpose' their 850MHz spectrum-slice for 4G users.
That will likely happen progressively, and quickly, given the recent 4G 700MHz commercial-approval granted to the other two telcos.
Unless Vodafone very seriously bolsters coverage on its 900MHz band, it's not hard to imagine that many 3G customers on (satisfactory-at-the-moment) mobile broadband and phone plans may find themselves with 'new' problems.
I have a 3G mobile-broadband plan on the Vf network that I maintain as a backup.
No 850MHz-enabled towers near me - that I'm aware of - at least by way of the coverage map I can remember looking at.
So, no skin off my nose as regards Vodafone's current plans.
Just a heads-up. May be different for you.
Just so you're aware, they're not killing 850MHz 3G..
They're planning to halve its allocation from 10MHz to 5MHz.
So you'll still get 850MHz 3G signal, but speeds may drop until people transition across to 850MHz 4G devices.