I moved everything to Vanguard Total Stock market ETF before we moved.....I believe those are a HRMC reporting fund.....which I did after the topic came up on this site.
I believe that you are correct that VTI (CUSIP number 922908769) is an HMRC reporting fund.
Since it is a joint account with your wife I would expect that both of you would need to report the dividends to HMRC from that fund plus the capital gains when you sold the fund. (one half each). My wife holds VTI so I know that it pays out quarterly dividends which should, I believe, be treated favourably as stock dividends.
We each recently applied on-line for self assessment to get a UTR number and the website said that if you receive taxable foreign income over £300 then you should be filing a self assessment return. I don't expect my wife to owe HMRC any taxes as the dividends she receives from her VTI and VYM funds are below her taxable income thresholds, but I believe she still has to file since she has foreign income.
ETA
I changed our joint accounts to her name before we left the USA to minimise the taxes since my US pensions put me in the 20% tax bracket.