Can I use Canadian visitor visa after event is over?

Yes, you can still use the approved Canadian visitor visa even if the original purpose (the conference) has already passed. A visitor visa allows you to enter Canada temporarily for tourism, visiting family/friends, or attending events, not just the specific reason you mentioned in your application.

You may travel to Canada for a vacation as long as your visa is still valid.

At the border, you should be prepared to explain your updated purpose of travel (tourism instead of conference) and show proof of funds, return ticket, and accommodation.

Yes, you can visit Canada for a vacation with that visa, provided you meet the entry requirements.