You could also take a look at generic Ruby tutorials too, especially if you want to familiarise yourself with how to program in Ruby. The difference between VX and VXA in regards to Ruby, is that VX uses Ruby version 1.8.1 and VXA uses version 1.9.2 which has a number of different features - most of which aren't taught or even shown through the RM help files (mostly additional methods belonging to the base classes like Object, Class and Module).
For the most part, if you know how to use RGSS2 you won't have any real trouble with RGSS3. The real task comes in having to look through the default scripts to see where the things in RGSS2 are in 3. Some things were kept the same, but there's a lot of things that were split up into new classes or moved to a different class found in RGSS2.
So yeah, first I'd check out Ruby tutorials and guides to get a good idea about programming, both specifically in Ruby and in general. Then, I would have a go with RGSS2 and try to implement your ideas with that especially the more simpler sounding ones.
Here's somewhere you can go to get some good information:
http://www.ruby-lang.org/en/This is the Ruby website that you get referred to by the RM Help file.
http://ruby-doc.org/docs/ProgrammingRuby/html/index.htmlAnd this one is a good read that tells you about Objects (which is a key topic you should understand) as well other general things like Variables and Classes and more Ruby specific things like syntax and code layout. If you go through this from start to finish, even if there's parts you don't quite understand (don't worry too much about using WINAPI with Ruby as that's a more advanced thing, for example) you'll probably have a good advantage for when you start scripting for VXA.