The vrm is already actively cooled on any waterblock that is properly fitted. Adding a thermal pad on the back wouldnt hurt anything at all, but it wouldnt provide much, if any, better cooling for the card.
The entire section on the left of the picture (next to the three standoffs) cools the mosfets, which is what gets hot. You should have put a thermal strip on there as shown in step 4 per the installation instructions. Did you skip that step, or did you use a different block? If you did not follow the instructions, you should definitely break it down and apply the thermal pads as required by the install guide.
Install instructions