Well, RMVX is made for developing a more eye-pleasing RPG. As you can probably see (if you tried the demo) that mapping and tilesets are handled differently, and there are really 5 layers now, rather than 3, and tilesets are back to having a limit. The framerate has also been boosted up to 60FPS, which is a nice touch. Now, handling the program as compared to RPG Maker XP is a bit different. As already stated, the mapping is done differently, and that tilesets are handled differently. Now, with the tilesets, you only have 5 tilesets, all of which appeared to be limited. The Events layer is also extremely inconviently plasced on the icon toolbar. One positive is that the Eventing system is bit more planned out, but it appears as if they left out some events (I don't have RMXP on this computer, so I couldn't compare). Now, the database is essentially the same, but some things are changed. For example, because of assumed tileset limitations, there is no tileset tab. They also have net different tabs for Weapons, Armor, and Items. Now, onto the scripting. Instead of using the same RGSS system as RPG Maker XP,they implemented an updated one, with slightly modified syntax and commands. This, of course, means that most RPG Maker XP scripts will not directly work with RPG Maker VX. Now, the Resource Manager was essentially the same, but since graphics are handled differently with the program, there were less folders than with RPG Maker XP. This included the tileset, which meant changing the tilesets needs to be done by hand. Now, the default battle system is the same as the one included with RPG Maker XP, which is a bit of a disspointment. A battle system like the one in RPG Maker 2003 would have been nice. The inclusion of facesets is a plus, but not somehing they should have prioritized when they were making the program.
Overall, RPG Maker VX is not really suited for Pokémon games. If you really want to upgrade, or if you have never touched RPG Maker, try the trial version and see what you thing. Otherwise, don't even bother with it. Personally, I am sticking with RPG Maker XP on my laptop.