VS Airbus planes are generally much, much newer than UA 747s but as BA304 says you're likely on a VS 747 at that time of year. Do a dummy booking and see what the VS web site tells you?
Have only had excellent cabin crew on VS.
Your seat can be changed by any airline but it is true that VS is changing its reservation system and while seat requests are available up to 90 days out currently, for your flight you won't be able to request a seat now (until the new system is operational).
VS does not bus to gates at Heathrow.