First of all, it's not Ruby and Ruby 2, but RGSS and RGSS2 (abbreviating Ruby Game Script Source). While they incorporate different base functions (all RGSS functions should be included in RGSS2), they're both built upon Ruby version 1.81 (currently established Ruby releases are 1.87 and 1.91, however, you won't be able to use them with RPG Maker VXP as far as I know).
Basically, the huge differences are in scene handling (VX has a Scene_Base that scenes are based upon, making XP>VX transitions easier), some new features (that sometimes make VX>XP conversions impossible without remaking those features, or using RGSS2 with XP), the different Battle, Menu and, most of them all, Map systems (especially for the latter, it's most likely easier to approach them from the beginning), along with a few minor things that you just need to know when converting (for example that $game_party.actors is called $game_party.members in VX, or really annoying-to-figure-out stuff such as $game_variables only holding integers in VX, while you could assign any variable type you wanted to in XP).
The help file gives you a neat insight on about what exactly's been changed (even though they don't list the details, which is the really interesting part :| ), so you might consult that one additionally.
Bottom line: The less your to-be-converted script messes with Scene_Map (and everything related, mostly Tilemap), Scene_Battle and Scene_Menu, the easier it will be to convert in theory. However, it's rarely automateable, meaning there's some effort to be put it to make this work usually.
