Hmm. Not sure I’d like to rely on Hoppitt-Levy for legal advice.

Their “quote” from the Vienna Convention of 1969: ‘withdrawing from a treaty releases the parties from any future obligations to each other, but does not affect any rights or obligations acquired under it before withdrawal’ – this actually comes from a House of Commons Library paraphrase, so it’s misattributed. Not only that, but it’s a selective (and thus misleading) quotation – it should be prefaced with the words “Generally speaking“.

More importantly, Article 70 of the Vienna Convention itself makes clear that this general principle applies only where the treaty itself makes no explicit provisions. Article 50 of the Treaty on European Union says:

3. The Treaties shall cease to apply to the State in question from the date of entry into force of the withdrawal agreement or, failing that, two years after the notification referred to in paragraph 2, unless the European Council, in agreement with the Member State concerned, unanimously decides to extend this period.

Otherwise it’s silent on whether rights and obligations to the Member State’s citizens would continue. Presumably it’s a matter for the withdrawal negotiations.