Moving Exchange mailboxed to New Domain

This sounds like it's in the same forest so you should simply be able to use Active Directory Users & Computers on either Exchange server to move the mailboxes.
Right click the user properties and select Exchange Tasks.

not sure if its actually the same forest as i am adding vdsarv4.volunteeering-ni.org after the other controllers. the new server does not appear in the vdasrv.volunteering-ni.org active directory.

You better had be in the same forest because you're in a lot of trouble if you have two AD forests on the network called "volunteering-ni.org"
There was something else that troubled me though. "vdasrv.vdasrv.volunteering-ni.org" is impossible as a server name because you can't have a server called "vdasrv" in a domain of the same name. I'm guessing you just missed a number there and everything is ok.
But yes, get someone to sit with you and confirm the number of forests on the network.

I currently have 2 servers
vdasrv.vdasrv.volunteering-ni.org and
vdasrv1.vdasrv.volunteering-ni.org
Things are OK apart from the fact that when vdasrv.vdasrv.volunteering-ni.org is down people cannot logon even though both servers are active directory DCs.

Then something is certainly wrong with AD. First off is the server naming thing I mentioned before and then secondly is what you said about the other server not appearing in AD like you'd expect.

I'd step away from the Exchange for a moment and do some work with Repadmin (http://support.microsoft.com/kb/229896) and DCDiag to make sure that your single forest (are we now saying you have a single forest?) is replicating properly.

Yes i want it to be a single forest.  I installed the new server as a new forest (volunteering-ni.org) which was probably the wrong thing to do.  
Can I add the new server to the existing forest at volunteering-ni.org level as even though the two existing servers are on the vdasrv.volunteering-ni.org sub domain.

No. You "must" reformat the box and install it all over again. (that's the safest thing to do, especially since there isn't anything on it you're going to keep.) Once you've reinstalled Windows, join it to the existing domain, reboot it and run dcpromo on it.

OK.   Will dcpromo bring the domain to the higer level?

That's a separate issue.
In Active Directory Users & Computers you right click on the domain and select the option to raise the functional level.

Sorry wrong choice of words.  I meant to raise it from vdasrv.volunteering-ni.org to volunteering-ni.org.

OK will do.  I will post back when I am ready to install DNS again just to make sure I do it right.

I have installed a new window 2003 Server on a new Forest.  
I have also manually recreated the AD users from the old server.  
I have installed Exchange 2003 and Exmerged to to the new server.  
I have tested loging on to the new server and all is well.

What i need to do now is
1) move my file server to the new forest. ( currently an AD DC)
2) transfer roaming profiles to new DC