The best way to learn linux is to use it. When I say use it, don't just load it up and surf the web, write a couple of text docs, and open up the terminal to see if it works in a GUI environment. Gentoo is a great distro to learn on, but its far from masochistic as stated. Gentoo compiles packages from source but uses portage and emerge, which is a package manager. The advantage is that you really don't have to worry about any dependency issues when you are installing something. The portage trees handle all that for you. Just like Apt does for Debian and Ubuntu. Depending on the installation method it can be challenging. Now there are some prebuilt installers for Gentoo (Kororaa, is a Gentoo based distro that uses install scripts), but you can really do your self justice by doing a stage 3 install using the hand book which has the step by step procedures.
http://www.gentoo.org/doc/en/handbook/handbook-x86.xml
The biggest thing I have found to really learn linux is to set up some sort of goal or scenario. For instance, give yourself an assignment to do a stage 3 Gentoo install, and then load up openssh, apache, mysql, and php. (Yes I know thats just a lamp server) But do them individually. Then once your done install some sort of CMS such as Drupal, Joomla, or Wordpress to make sure its functional. Treat it like a school assignment or a work assignment. Give yourself a deadline. Make it like a game even. Once you have accomplished that.. Do it again. Maybe this time once you have installed the OS and openssh.. do the rest strictly over SSH from the a different box over the network, or I guess in your case from the VM host machine
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If you want even more of a challenge.. then its best to begin with something a little closer to when it started. Slackware!
http://www.slackware.com/
One of, if not the oldest distro. Everything is compiled from source. Downloading and extracting tar balls, setting up the .config file, running a make on it and praying to the kernel gods.. ahhh.. the good old days..
I honestly don't miss them! But I did learn a lot.
Hope this helps, and good luck on your endeavor!