Hello, this is Jim and I am a dual-qualified lawyer (UK and Republic of Ireland) and happy to help you this evening.
Is the main question whether you should dispute paying the £400 to Vodafone?.
You can allege breach of contract against Vodafone (you have consumer rights here which have been infringed in my view).
Given the circumstances of your case I recommend that you make a formal complaint against Vodafone and if that does not resolve the matter ask them for a final response letter (a "deadlock" letter).
You can make the complaint with Vodafone here:
Once you have the deadlock letter the next step would be to escalate this to the adjudication scheme (CISAS) - they will investigate and liaise with Vodafone which would hopefully result in a refund of any monies paid. They can also order the phone company to make a financial award if they have acted badly.
The service is covered by the Consumer Rights Act 2015 so you have a right to expect a reasonable service - if they do not provide that then the regulator will step in but it also means you can sue for any damages or losses incurred as a result of Vodafone's conduct.
You can escalate to the adjudicator here : https://www.cedr.com/consumer/cisas
If you have lost any money then you could sue Vodafone but the court may consider that to be premature if you do not go through CISAS first of all.
You can sue for your losses via the http://www.moneyclaim.gov.uk website which is easy to use and no lawyer would be required.
I hope this helps - if you can please accept the answer and give me a 5 star rating (there should be a button at the top of your screen to do this), I can answer follow up Q&A's at no extra charge and Just Answer will credit me for helping you today.