Warning when adding multiple VirtualHosts on port 443 - "_default_ VirtualHost overlap on port 443, the first has precedence"
You need to add NameVirtualHost *:443 for it to work.NameVirtualHost *:80 must also be in your configuration, it is probably set by default, but I don't use Ubuntu so I'm not sure in which file.
Also a word of advice:
Running multiple HTTPS sites on a single IP might have it's drawbacks, specifically older clients (IE8/Windows XP) will not support SNI.
It may make perfect sense in your situation (only for testing, *.example.com wildcard certificate is being used, etc.) but you should be aware of this.