I might have missed this in an earlier posting, plus I've never yet played with the vpsupd specific to v5 (wasn't even sure it was available yet), so this may be totally off-base. On the offline computer, do you need to temporarily disable the self-protect thing while running vpsupd? You didn't in 4.8, but of course there's been major changes to the way updates work in 5.
I'm in essentially the same position, my wife's computer doesn't have internet access and I've been using the 4.8 vpsupd once a month or so on hers. Haven't yet upgraded hers to 5, but of course without internet access her likelihood of running into malware is much lower.
I'd like to see a "sticky" on this matter (offline updates), since obviously there's bits and pieces of info about it all over the forums which don't fit neatly together.