how to convert a Sync-on-green to VGA (seperate sync) signal?
There are ways to do what you want.  Depending on your abilities and budget they may or may not be "easy"...

There are different video converter boxes which take video input in one format and output it another.  For example see: Grass Valley's Video Conversion Hardware  This was the first link which appeared in a Google search.  I'm sure that there are plenty of other options and many will likely be cheaper as well.

Another option which doesn't involve buying something, would be to build you own circuit.  I'd suggest basing it off of the LM1881 sync stripper IC.  LM1881 Sync Stripper  This is a pretty solid IC and works pretty much out of the box.  The application notes are pretty clear and its application is very straightforward.  You can buy it for a few dollars and its available in both surface mount and through hole so its easy to build with.  Just add a couple Rs &amp; C's and you've got the sync signals.  Buffer them with an amp/driver and you're good to go.  You won't need to actually remove the sync from the green signal since it won't hurt to be present.

Good luck!