As you may be aware Yammer ‘on by default’ now as part of Office 365. This means you *may* get a Yammer network provisioned at the *.onmicrosoft.com
domain associated with your tenancy.

Here’s a tip
If you want to take advantage of this make sure that you create the tenancy as a Trial and then buy licences later. Taking this approach means you get the Yammer ‘on by default’ experience more or less as you would expect, it will be provisioned within a hour or so (maybe sooner).
If you instead purchase a tenant right away, they apparently wait a while as the expectation is that you’ll want to use a vanity domain for your Yammer network… “But I want Yammer ‘on by default’ with my paid tenancy”! You can create a Yammer network on your default Office 365 domain just by logging into Yammer with your *.onmicrosoft.com
account BUT you don’t get the integrations with Office 365 – the app launch app, the link to Yammer admin from admin centre. Based on my experience, you will get these integrations eventually – I saw them appear more than 6 weeks later!
I had a long call with Microsoft support regarding this and they weren’t able to provided any solid explanation or reasoning; the information contained in the post is based on my experience rather than official guidance.
Summary
Create new (dev/test) Office 365 tenancies using a Trial subscription and buy licences later if you want to use Yammer without a vanity domain.
Paul.