Jumping in to point out that Susan is a typical character archetype in British children's adventure stories. Just like Peter is the Responsible Older Brother figure, Susan is the Motherly Nurturing/Nagging Older Sister. She shows up a lot in stories of four or more children having adventures, usually under the name of Susan or Ann or Jane. She's the one who says things like "Don't forget to wipe your face" and "But Mother said!" and "We have to be home by dinner."
She's also the one who handles practical things, though, like making sandwiches or grabbing the mittens you forgot, and she may double as the Caring Voice or Emotional Center--comforting people, that sort of thing. Nesbit usually has at least one girl like this, and Jessie from the Boxcar Children is pretty similar. In Narnia, Lucy often takes on the comforting role, but Susan does the nurturing and the practicalities. This isn't even always a bad thing--remember, Su and Peter both worry about Lucy ("Should we write to Father?"), and she does things like remind people not to lose their shoes on the beach. The flip side of her personality is that she hates striking out into the unknown, and this shows itself in PC (although even there, it's only Lucy who immediately sees Aslan).
I think you have to read LB to see Susan going in that direction in VDT--being pretty and not too good at schoolwork doesn't make her materialistic any more than being smart and funny-looking makes someone a good person. And her parents seem to be using pretty standard parental logic there ("Peter's at school, so we should bring the oldest, who is almost finished with school and probably ready to start looking for a husband or something to do with her life, instead of the two little ones who wouldn't really appreciate the trip!")