I have used \"alias ruby=ruby1.9.1\", so I can execute my ruby with this:
ruby 123.rb
or
ruby1.9.1 123.rb
When Vim starts a process it makes a system call. It has only inherited the environment variables from your shell if you started it from the shell. But it won't know your bash aliases.
Bash aliases are only a convenience when you enter a command line in the Bash shell. They are expanded by Bash only.
If you want real aliases put symlinks in a private hidden folder, and add that folder to your PATH, or use the alternatives facility.