From a quick read around I can’t identify a requirement for a further vote in order to crash out (if anyone else can feel free to point to it!).
The triggering of article 50 itself was done by act of Parliament (as was required following the Gina Miller case), authorising the prime minister to deliver the letter triggering the process.
Article 50 itself (an EU treaty provision) is quite vague and simply confers a two year timetable after which the treaties will no longer apply to the departing member state. The mechanism for departure is (obviously) a matter for the constitutional requirements of the member state concerned.
The European Union (Withdrawl) Act 2018 is the substantive piece of legislation which repeals the European Communities Act 1972 and recasts much existing EU legislation into domestic law. When this legislation was tabled the bill was amended to give parliament a “meaningful vote” on the deal.
(Paraphrasing here, but I’ve linked to the act below and suggest following the link - apologies it’s impossible to copy and paste in full).
Section 13 introduces the requirement that parliament must ratify the eventual deal. Interestingly the same section provides for the situations where:
(a) the PM states before 21 Jan 2019 that no agreement can be reached on arrangements for withdrawal and framework for future relationship;
(b) there is no agreement in principle on 21 Jan 2019 which can be put to the commons;
(c) the commons rejects the proposed deal.
In each of these eventualities the requirement is merely that a minister makes a statement of how the government intends to proceed. This statement is required to be endorsed by parliament by a “motion in neutral terms”, but no vote is actually required.
So if there is no deal or the deal is rejected by the commons, it appears that there’s no requirement for a vote to be taken in order to crash out. Whether that would actually happen of course is another matter.
What might then happen is anyone’s guess. It’s also worth noting that it’s by no means certain that the EU’s legal system would permit it to agree an extension of the article 50 timescale.
http://www.legislation.gov.uk/ukpga/2018/16/section/13/enacted