Forget Vagrant. You don't need it, should never need it and at this stage I would suggest adds too much of a learning curve.
Dual boot is the best option, installing Rails is a lot simpler on Linux than it is on windows. Whilst the software manager is your friend (Sort of an app store) Don't install Rails from the software center. Use RVM and follow the instruction in the book. For most things when you are looking up how to do stuff in linux Ubuntu and Mint instructions are interchangeable. Mint is a derivative of Ubuntu.
As an alternative to AWDWR instructions, these ubuntu instructions are up to date (at time of writing) and are dead simple to follow.
I feel for you with regards to Delphi and Visual Studio. I have used both in anger in the past although too many years ago now. You have no option other than to keep windows for these, however, they may work well in a virtual machine under Linux with Virtualbox http://lifehacker.co...n-app-for-linux which again can be found in the software center.
Your need to go hunting around the net for software will be dramatically reduced and 99% of the doftware you install will be free open source and a real eye opener.
You will find virtualbox in the software center.
You have a lot to get your head round but you will find Linux a true eye opener. Mint is dead simple to install. Use a live CD/DVD, just download an iso image from the mint site http://www.linuxmint.com/download.php and burn to DVD, then boot up with the DVD and you will be able to have a play with Linux before installing it although anything you do will not be saved until it is installed. Once you have finished playing there is an install link on the desktop, when asked what format to use for the file system choose ext4
The installation process will automatically detect that you have Windows installed and will ask if you want to keep it and will automatically set up a menu for you the next time you switch your P.C. on to choose if you want to boot into Windows or Linux.
The actual installation process is very quick compared with a Windows install, no licence keys to worry about, but make sure you pick a secure but memorable and more importantly easy to type password. When doing potentially destructive actions in Linux such as using sudo or installing software you will be asked to type your password.
Warning, not all Linux distros are this easy to install, if you choose to go with a different distro (arch for example) then you really have to know your onions. Debian based distros or more importantly Ubuntu spin offs are all dead simple to set up.
With Linux unlike windows, where Windows is the desktop, you have a choice of desktops, in fact there are so many different desktops to choose from it is mind boggling, stick with Cinnamon or Mate until you feel ready to try some others out.
I know you didn't ask for pointers on Linux but there is no point me recommending you go with something you are unfamilar with and leaving you in the cold!
If you need any further help just shout