Hi there,
I use the E-
juice me up software available
here
Within this software is a "nic combine" tool and with some trial and error on that tool you could find out how much ejuice you would need to add to reduce the nicotine to your preferred level. Also you could use it to calculate the VG needed to raise the VG content however I'm not sure how much flavour would be left after diluting the juice.
There may be a better solution but I'm offering what I know and have used myself. I hope it's helpful to you.
Using that tool I calculate that adding 60ml of 100% VG to 30ml of what you have would give you 90ml of 73% vg 27% pg ejuice at 6mg Nic