Interesting, (well nearly), I caught a bit on the BBC parliament channel earlier, it was the house of lords which I'd usually not bother with, but by chance a lady was talking about something which had been touched upon previously within a brexit debate before the meaningful vote.
Thankfully, it clarified a couple of things for me: article/section of the Vienna convention, would probably mean that the EU couldn't keep the UK in the backstop.
Moreover the WA wouldn't override the Vienna convention, because of various clauses.
The lady concerned explained that a number of top international lawyers were of that opinion, and a number weren't.
Thus it would end up in very long-winded legal deliberations. As she put it "this should focus minds of the EU" with regard to current negotiations.