I suspect you're thinking about this in slightly the wrong way.
The repeat control is automatically looping over the elements in the vector for you. You can output each element of the vector by referencing the variable name the you used when creating the repeat control (the collection name).
If you've got a check box being repeated then you'll need to have a read up on check box groups here: http://www-10.lotus.com/ldd/ddwiki.nsf/dx/02102008091536PHABPL.htm and then if you want to access that value you can either bind the checkbox group to a document binding or a scoped variable in the data settings.
Hope this helps.
Matt