Continental use Air Canada's Arrivals facilities at Heathrow because they don't have their own there (I could be wrong, but I think that most/all of the CO's flights on CO metal fly from LGW in the UK - CO have their own Arrivals facilities at LGW).

I guess that CO uses Air Canada's lounge instead of VS's because either VS do not want any other passengers using their lounge (capacity/exclusivity) OR Air Canada's lounge is a cheaper option for CO. But that would be purely speculation....

CO codeshare on VS (and I assume VS) use Air Canada's departure lounge at LAX: again I assume that CO does not have a lounge in the terminal for VS departures.

I am just grateful that CO passengers on the Virgin codeshare get to use the Virgin Clubhouse at LHR. If the Virgin check-in agent offers/gives you passes to another departure lounge (this happened to me at LGW during the Summer), ask if you can use the Virgin Clubhouse instead. You are perfectly entitled to. I forgot to ask for lounge passes to the clubhouse instead (they gave me CO President's Club passes), but they welcomed me at the LGW Clubhouse anyway.

Different start and destination points, but my UC CO/VS code share trip report is on the following link (might be helpful):
www.flyertalk.com/forum/Forum81/HTML/002066.html

I could be very surprised, but I think that you will have a great flight: Virgin lounges are great, their planes are very comfortable, the food is good and very flexible and the service is very attentive and personal. I can never understand why people would fly business class on a proper three class carrier... (but then I haven't flown business class on a three class carrier for ten years... so may be badly informed)

Boo
(hoping that she never has a negative experience which would decrease her complete satisfaction with VS)