The 10MHz blocks are to be auctioned first. If Optus wins the...

The 10MHz blocks are to be auctioned first.
If Optus wins the 10MHz block, it will cause a lot of pain for Vodafone...
As stated in the article it is not worth building a network with just a 5MHz Block
Optus have been throwing a lot of money around lately and have 1000 more 700 MHz towers than Telstra!
Optus need this spectrum for speed and capacity to compete with Telstra..

If Optus wins they earn the ability to compete 1:1 with Telstra.
and they will dramatically increase the build costs of one of its competitors, Vodafone, as building a 700MHz network means less towers due to its better range.
If Optus loses they will always have an inferior network and will need more towers or have a slower network.
So its high stakes for them too.

Vodafone have been pushing domestic roaming extremely hard...
Is this related to the spectrum auction?

If Vodafone don't get domestic roaming or a 10MHz block of Spectrum they are somewhat stuffed in Australia.

Reading the submissions to the ACCC my leaning would be that domestic roaming wont proceed.
It is opposed by Telstra and Optus, and probably 70% of non related interest groups, that have provided submissions.

Vodafone must win the 10MHz Spectrum block.

How nasty does Optus want to be?