That rt. rear bolt is the hardest to get to, I think. And that bolt pattern is pretty specific and makes a difference.
Then there's the torque issue...I've said and mostly still sort of believe, that up until the time the screw hits the head, the torque only comes from compression of the grommet and that increases very slightly as one tightens down a vc with new gasket. I'd learned 'down to head' and then stop. Mango has said he goes to full stop and then backs it off a few turns (I'm pretty sure).
I have noticed that my vc seems to be bent up a little, but as all my screws are down to the head, I attribute this to the vcg I used, which seems to sit proud of vc just about 1-2 mm more than my old one did (my brittle one which I did at 180K or so).
I'm thinking that a vc could get bent, like mine, and still seal fine with even torque...
Anywhoooo...have you tried just tightening that one corner bolt or going through the pattern again down to when screw stops...in slow stages. Might get lucky!