ok it does not double it for application open however, it does increase it substantially per application open, for 2 applications open it is double the memory usage of 1 application, for 3 applications open it triples the memory usage of 1 application open, 4 applications open takes a lesser hit, but after that it is back to the normal trend. other os's windows 7 for example uses little to no extra memory per application open, and xp is similar.
I'm sorry i cannot agree with you that vista is an excellent OS, i use it at work and its the worst experience I've ever had in my life.