VS does not operate Economy + just Economy and Premium Economy. On some planes 54H is an extra leg room seat in Economy so it will be the same economy service as the rest of the cabin you just get a few extra inches of leg room.

If you are downgraded you are entitled, under EU261, to reimbursement of 75% of the fare (excluding taxes and airport fees etc) for the flight sector downgraded.

If there is any difference in taxes then you will get all of the difference back so flying LHR-MIA you would get back £75 of the APD you have paid.

On the day (possibly even before) VS will ask for volunteers to either downgrade themselves or to rebook onto other flights. This will involve you been offered a lot of VS miles or a free flight at a later date (as well as rebooking for the current trip)