How can I seal a roof vent to resolve drafts in my home?
You apparently haven't asked yourself why that's there in the first place. Attic ventilation is a critical component of a healthy home, and you should not block it without good reason. A bit of errant of snow is not a good reason, nor is flawed sealing of your home's livable space. 

Your habitable space should be sealed completely from the outside world via your ceiling, not your roof. Find what's leaking in your ducts or the penetrations they require and deal with that to solve your problem. That may require moving insulation to foil-tape or caulk duct joints, and adding spray foam or other sealer around where they pass through the ceiling.

It's not uncommon for a fine dusting of snow to be blown in past attic vent screens on occasion, which isn't cause for alarm. It'll sublimate (evaporate) away over time. Now, if it's a half inch or more thick you probably want to upgrade the vent housing to a better or more suitable design.