Owch. holy crap! don't do this! That liner over a bag will leave you soaked and cold.

The only thing you should consider is putting such a thing as a vapour barrier *inside* the bag, not overtop! with something like that over the bag you'll just wet it out in many situations.

Now, one practcal alternative for the OP instead of this is to
buy themselves a lighter (like a 20F or 32F) good quality down bag, and then leave a couple of options

1) put the down mummy inside a synthtic barrel bag (cheapie from wal-mar) when colder,
and/or
2) learn to sleep in the bag with some layers on.

I sleep in -30C - and I use two bags. I don't own a -20 sleeping bag.