Concerning converting VX Ace projects to MV:
1. Graphics --- you'd need to use a tool to resize tilesets to 48x48. I've heard waifu mentioned. It lives here:
http://waifu2x.udp.jp/ and resizes your tiles up 2x normal. It uses a neural network to resize JRPG styled art, so it's well suited to RPG Maker VX Ace tiles.
Then use GIMP (which is free) to size the tiles down to 48x48.
2. Audio --- there is a free tool called freeac here
http://sourceforge.net/projects/bonkenc. This says it will convert OGG files to M4A, so they can be used on mobile.
3. Maps --- I think Shaz kindly made a script available, which is in the Steam pre-order bundle, which converts maps to the new JSON format used by MV.
4. Custom Scripts --- This is the trickiest part. Plug-ins do the same thing but any custom scripts will need to be rewritten in Javascript to work with MV.
Those should help ease the transition.