I think strictly Vanguard are wrong about the ROTH if you have a SSN and live in the UK as ROTHs are tax free to UK residents, but sometimes it's just too much work arguing with fools.
I fully agree with you, and I had 2 email discussions with 2 different advisors. It wasn’t a problem for me as I simply cancelled my planned and scheduled quarterly tax payments. I now have no quarterly tax payments but also have no Roth withholdings since I changed the correspondence address to a US address. When you look at my account on Vanguard it has a street address in England and a correspondence address in the USA. The wording on the website is clear that the street address is where I reside but if I have a US correspondence address then no withholding.
When we first moved back the street and correspondence were also UK and USA (our daughter’s address in California) but a copy of the 1099-DIV from the taxable account went to the California department of revenue who came after us for California taxes and I had to send in proof, including a copy of our federal return, that we not resident in California and that Vanguard was not a California based company.
When you get back and start filing tax returns with HMRC you are going to find it SO MUCH easier and a lot less hassle than the IRS and State returns. I still have to file and pay taxes to Louisiana because I have an unqualified pension (a SERPS) paid and reported on a W-2. The IRS insists on FICA taxes, and Louisiana insist on a non-resident return and taxes because the company paying the SERPS is registered in Louisiana. I haven’t set foot in Louisiana since retiring in 2010.