I did the VT 50 last year.
I ride a singlespeed MTB most of the time but keep a geared bike for racing.
from my recollection I say that you will be better off running most of the short steep climbs in the VT 50. and that most of the longer climbs are on jeep track/dirt road or smoother singletrack.
As far as Climbing on a singlespeed. For me, I try and stay seated for as long as possible to keep momentum and to keep rear tire traction. Once you get out of your seat, torque the hell out of those bars and then you gotta know when you are beat so that you have enough momentum left to dismount and run up in a nice motion. Once you spin out on a loose rock or wet root, I am usually done on my SS.
Like Born 2b said, with running, most times you will be passing geared bikes in there grannys. A brisk walking pace will be granny gear speed.
So, in summary, I offer you no help at all. Good luck to you and I'll see you up there this year on my Dos Niner.