It's awesome you want to help development of the this magnificent text editor: you can help in two ways I can think on top of my mind, if you browse the www.vim.org site, I'm sure you will find more ways.
1. it's open source project: donate your development time, write documentation, develop useful scripts, anything you can really...
2. you still can donate money, regardless of helping the children in Uganda, because "donations increase Bram's motivation to keep working on Vim". That's a good cause, wouldn't you agree?
It's called the free market.