1) Make a second R and line it up exactly with the one you have. (If you cant'
get perfect alignment, make a copy of your text, convert to path, ungroup, and
delete all the parts except the R.)
2) Make a copy of your vector shape.
3) Select both the copy R and copy vector shape and then Modify>Combine
Paths>Intersect.

What this should give you is just the area where the R and the shape both
overlap. Then....
4) Change the color of this area to whatever you want it to be.