I went ahead and wrote up a quick CG guide for you! It's divided into sections just like the CG gallery is in game, and it has a description on how to unlock every CG.
Each description also links to a picture of the CG used for the preview in the CG gallery. This is just in case you need to visually compare the CGs in your gallery that you have/haven't unlocked with the descriptions in the guide. If you haven't unlocked the CG in the game yet, clicking the link for it in the guide will show you the CG.
You should've been able to unlock the bottom one on the left row (#7 on the CG guide for Vincent), if you unlocked the one right above it, the 3rd one on the left row (#5 on the CG guide for Vincent). I just did a fresh play through of Vincent's route going for the bottom left (#7) CG, as well as checking the code for unlocking that CG, and it unlocked in my copy. If you have trouble unlocking that CG after reading the guide and playing DFU again, let me know. Edit: I was looking at an older version by accident. The CGs for #5 & #6 have switched places since that version. I'm sorry!
I hope this CG guide helps! Have fun with completing your set!^^