Could you give a bit more explenation to what your trying to accomplish because I don't think I'm understanding you proper.
In any case I think this might be of use from what I could gather:
Well it being in a VM Environment shouldn't matter, unless your trying to keep it all secluded from the "live" network you are on.
If you want to keep it seperated from your "real" network you'll need to use VLAN's inside of VirtualBox (configure the virtual network adapters it creates), everything else should be explained I guess in the above tutorial.
Or do you require further aid with getting started and all as well ?
How have you configured your Virtual NIC's ? maybe include a picture if you can / screenshot of setup. Sounds like you are putting the Virtual NIC on the same subnet as your "real" network and its giving conflicts.
Anyways I actually wonder why your doing this.
If all your doing is setting up a LAMP server that gets redirected to with the usage of DNS then just setup a BIND server as a DNS server who forwards everything to your school / home / Company Network, and add specific zones strictly and only for the Domain you want.
Point your own DNS server settings to there and voila, your VM will forward all DNS queries it doesn't personally have (so everything that is not yoursub.yourdomain.tld) to the existing servers.
its a fairly simple solution to your needs, and isn't too hard to configure as opposed to starting to setup all kinds of Virtual Lans
Addendum nr: 3
I have a tutorial / guide somewhere that I wrote myself once but can't find it, for now I'll link the Ubuntu documentation:
Its the section where they talk about forwarders.
I'll take a look tonight when I'm off work and if still needed I'll put the tutorial available somewhere.