I don't know about the rest of RMRK, but I personally don't care for the chibi style that VX has to offer, so I set out trying to find a way to use my good old "2-tile" character sets I had come to know and love.
To begin, export a your favorite character sprite from RMXP. I used 001 - Fighter01 just because it was first in the list.
Not adding a screenshot for this. If you don't know how to export, lemme know and I'll send you a PM.
Next, open the Character sheet in Paint. I'm sure other image programs can do everything here, but all I have is paint.
We need to remove the first 32 x 192 pixels from the image. Rather than try to cut out these pixels and then move the whole image, we're going to cheat a little. Go to Image > Flip/Rotate, then hit OK.
Now the first column is the last column. Yay! Next, go to Image > Attributes and set the width at 96 [128-(128/4)]. The first column is gone for good.
Now we have to flip him again, the same way as before. If we don't, he'll be walking backwards all the time.
Now, open attributes again. VX has to have 8 different Characters on a character sheet, so we'll use the attributes window again. Set the image to 384 x 384. The size of the characters in Xp make this a perfect square. Make sure the character background and the background you just added are the same color, and voila! You just converted a Character!
Be sure to save it in .PNG format, then import and choose the transparency.
In case you're lazy, busbuzz has already converted a hell of a lot of characters, and has a thread
Here. I just thought that the people should know how to convert their own.
Yes, I used Paint on Vista, but all the features are the same as XP.