So this really depends on the level of skill.
If your PC is in a "Homegroup" or "Workgroup sharing" state, then yes, he can very easily get to your Data.
Router logs are useless with a VPN as all traffic is pointed to the VPN's listening port.
For the 'packet sniffing' part of that question: When you establish a VPN connection, it should be encrypted from source to destination. The type of encryption is going to largely determine the ability to read the data. Since you're communicating wirelessly, everything you do is being broadcast around the house. Encrypting that will help, but again comes in the level of skill. He COULD perform a "Man in middle" attack, or capture packets and then replay them, or simply capture data using a sniffer and read the data. Some VPN providers are still on MD5 packet encryption. BackTrack cracks MD5 hashing very quickly. The question you really need to ask is; are you sending banking data over the network? Are you rich? Is there any benefit to him seeing what you're doing?
When hackers get good enough, they typically don't waste time on trying to see the same news you did, they want money or to play tricks. If you're worried about him stealing from you, then I wonder why you share a house with him.
What you need to do:
Startup in safe mode and do a full Virus scan using two "On Demand" scanners.
Set a good password on your PC (both your account and administrator account) and disable the administrator account if you're not using it.
Disable any type of sharing and change your workgroup name to something non-default
Disable RDP connections and assistance
See what kind of encryption your VPN offers and keep the VPN on at all times.
Encrypt sensitive files (like a document that has passwords or financial data) with something like axcrypt.
Lock your door when you leave - Physical access trumps everything!
Look for a new place to stay if you're getting suspicious of your roomy.