If SW have closed it wouldn't it just be a case of restarting the workplace pension from scratch? As in you're just 'opting in' again. So you would have a workplace pension, plus the Vanguard SIPP.
If that's the case then you'd probably just lose a couple of months conibutions whilst it's set up.
not always that simple - if you opt out of a workplace pension, they (employers) do not have to accept you back into their workplace scheme if you’ve opted in and then opted out in the past 12 months.
However - Your employer will automatically re-enrol you in the scheme. They must do this either every 3 years (from the date you first enrolled), or they can choose to do it sooner.
Depends whether this is classed as "opting out" or "transfering out"