Vim script to replace all occurrences of text using a REST service

Vim script to replace all occurrences of text using a REST service

I am a long time vim user but have never written a plugin before. I would
like to write a pluging to replace all occurrences of a predefined tag
using a REST service. So if I have defined the tag to look like <>, the
call a REST service that takes an argument "foo" and passes back bar. So
if I had the following text:
Lorem <<lookup: foo>> ipsum dolor sit amet,
would transform into:
Lorem bar ipsum dolor sit amet,
I need to know how to do the following:
Call a REST service from within vim script
Parse the REST response
Parse a file looking for the tag I've defined so that I can call a
function using that tag as a variable to the function.
I'm a java programmer but vimscript is mostly new to me. Any help would be
appreciated.